Class 9 Chemistry — Important Questions with Answers

48 concept-first Class 9 Chemistry questions across 4 chapters — each with a clear model answer, the why behind it, and a memory trick to make it stick. NCERT-aligned and free. Build the base now, and your board exams, NEET and JEE feel a whole lot easier later.

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Matter in Our Surroundings (9)Is Matter Around Us Pure (11)Atoms and Molecules (14)Structure of the Atom (14)

Matter in Our Surroundings9 questions

ChemistryEvaporation and coolingmedium

Why does evaporation cause cooling?

Reveal answer▾

What it is

When the fastest particles escape a liquid's surface, the particles left behind are cooler.

Answer

During evaporation the fastest-moving (most energetic) particles at the surface of a liquid escape into the air. This lowers the average kinetic energy of the remaining particles, so the temperature of the liquid falls. The escaping particles also absorb latent heat from the surroundings, which is why evaporation produces a cooling effect - for example, sweating cools our body.

•Fast (energetic) particles leave the surface
•Remaining particles have lower average energy
•Latent heat absorbed from surroundings
•Example: sweating cools the body

Why learn this

It's why sweating, earthen pots (matka) and desert coolers keep things cold without a fridge.

💡 Memory trick

The 'hot-heads' leave first, so what's left behind is cooler.

ChemistryKelvin temperature scaleeasy

Convert 25 C to the Kelvin scale. Slide the temperature to explore.

Reveal answer▾

What it is

The Kelvin (absolute) temperature is the Celsius temperature plus 273.

Kelvin = Celsius + 273

Temperature25 °C

Temperature298 K

Answer

Kelvin = Celsius + 273 = 25 + 273 = 298 K. The lowest possible temperature, absolute zero, is 0 K, which equals -273 C; that is why the Kelvin scale never goes negative.

K = C + 273

•K = C + 273
•0 K (absolute zero) = -273 C
•The Kelvin scale has no negative values

Why learn this

Gas laws and scientific work use the Kelvin scale, which has no negative values.

💡 Memory trick

K = C + 273. To go back, C = K - 273.

ChemistryKelvin to Celsiuseasy

Convert 300 K to the Celsius scale. Slide the temperature to explore.

Reveal answer▾

What it is

To convert a Kelvin temperature to Celsius, subtract 273.

Celsius = Kelvin − 273

Temperature300 K

Temperature27 °C

Answer

Celsius = Kelvin - 273 = 300 - 273 = 27 C. So 300 K is a comfortable 27 C. This is simply the reverse of the rule K = C + 273.

C = K - 273

•C = K - 273
•Reverse of K = C + 273
•300 K is about 27 C

Why learn this

Everyday temperatures are in Celsius, so we often convert back from Kelvin.

💡 Memory trick

C = K - 273. The reverse of K = C + 273.

ChemistryStates of mattereasy

How do the particles change as a solid is heated into a liquid and then a gas? Slide the heat to explore.

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What it is

Matter exists as solid, liquid or gas depending on how much energy its particles have and how tightly they are held.

States of matter

Heat / temperatureSolid

Particles are packed in a fixed pattern and only vibrate. A solid keeps its shape and volume.

Answer

In a solid, particles are packed in a fixed pattern and only vibrate, so it has a fixed shape and volume. Adding heat lets them break free into a liquid, where they stay close but can slide, keeping the volume but taking the container's shape. More heat spreads them far apart into a gas, which moves freely and fills all the space.

•Solid: packed, fixed shape and volume
•Liquid: close but flowing, fixed volume
•Gas: far apart, fills all space
•Heating increases particle energy

Why learn this

It explains melting, boiling and why gases fill any container.

💡 Memory trick

Add heat -> particles gain energy -> solid to liquid to gas.

ChemistryStates of mattereasy

Compare the three states of matter in terms of shape, volume and compressibility.

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What it is

Matter exists mainly in three states - solid, liquid and gas - that differ in shape, volume and compressibility.

Answer

A solid has a fixed shape and a fixed volume and is almost incompressible, because its particles are closely packed with strong forces. A liquid has a fixed volume but no fixed shape (it takes the shape of its container) and is only slightly compressible. A gas has neither a fixed shape nor a fixed volume and is highly compressible, because its particles are far apart with very weak forces.

•Solid: fixed shape and volume, incompressible
•Liquid: fixed volume, no fixed shape
•Gas: no fixed shape or volume, highly compressible
•Particle spacing and forces decide the state

Why learn this

It explains why ice is rigid, water flows and steam fills any container.

💡 Memory trick

Solid = fixed shape and volume; Liquid = fixed volume, no shape; Gas = neither fixed.

ChemistryCharacteristics of particles of mattermedium

State the main characteristics of the particles of matter.

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What it is

Particles of matter are very small, have spaces between them, are continuously moving and attract each other.

Answer

The particles of matter are extremely small in size. They have spaces (intermolecular gaps) between them, which is why substances like sugar dissolve in water. They are continuously moving, that is, they possess kinetic energy that increases with temperature, which causes diffusion. They also attract each other with a force of attraction that holds them together, being strongest in solids and weakest in gases.

•Particles are very small
•Have spaces between them
•Are continuously moving (kinetic energy)
•Attract each other (strongest in solids)

Why learn this

These ideas explain diffusion, dissolving and the states of matter.

💡 Memory trick

Tiny, spaced, moving, attracting - the four key facts about particles.

ChemistryChange of state and latent heathard

What is latent heat of fusion? Why does the temperature stay constant during melting?

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What it is

Latent heat is the heat absorbed or released during a change of state at constant temperature.

Answer

The latent heat of fusion is the amount of heat required to change 1 kg of a solid into liquid at its melting point without any change in temperature. During melting, the heat supplied is used to overcome the forces of attraction between the particles and to increase their spacing, rather than to raise their kinetic energy, so the temperature remains constant until all the solid has melted.

Heat = mass x latent heat (Q = mL)

•Latent heat = heat for change of state, no temperature change
•Fusion: solid to liquid at melting point
•Heat overcomes forces between particles
•Temperature is constant during the change

Why learn this

It is why ice at 0 degrees Celsius cools a drink better than water at 0 degrees Celsius.

💡 Memory trick

Latent heat is 'hidden' heat - the temperature stays constant while the state changes.

ChemistryEvaporation and its factorsmedium

What is evaporation? State the factors that affect its rate.

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What it is

Evaporation is the change of a liquid into vapour at any temperature below its boiling point, occurring at the surface.

Answer

Evaporation is the process by which a liquid changes into its vapour at any temperature below its boiling point; it takes place only from the surface of the liquid. The rate of evaporation increases with an increase in surface area, an increase in temperature, an increase in wind speed, and a decrease in humidity. Evaporation causes cooling because the fastest particles escape, taking energy with them and lowering the temperature of the remaining liquid.

•Liquid to vapour below boiling point, at the surface
•Faster with more surface area and higher temperature
•Faster with more wind, less humidity
•Evaporation causes cooling

Why learn this

It explains why wet clothes dry and why sweating cools the body.

💡 Memory trick

Evaporation increases with more surface area, higher temperature, more wind, lower humidity.

ChemistryEffect of temperature and pressure on statesmedium

How do temperature and pressure affect the state of matter?

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What it is

Raising temperature or lowering pressure can change a solid to liquid to gas; the reverse can liquefy gases.

Answer

Increasing the temperature gives particles more kinetic energy, so they move apart and a solid can change to a liquid (melting) and a liquid to a gas (boiling). Decreasing the temperature does the reverse. Increasing the pressure pushes gas particles closer together and can turn a gas into a liquid, while decreasing pressure allows particles to move apart. This is why gases such as LPG are stored as liquids under high pressure.

•High temperature: solid -> liquid -> gas
•Low temperature: reverse changes
•High pressure can liquefy a gas
•LPG stored as liquid under pressure

Why learn this

It is how gases like LPG are stored as liquids under pressure.

💡 Memory trick

Heat -> particles move apart (solid to gas). Compress + cool -> gas turns to liquid.

Is Matter Around Us Pure11 questions

ChemistryMixture and compoundmedium

Give two differences between a mixture and a compound.

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What it is

A mixture is a physical blend you can separate; a compound is a fixed chemical union with new properties.

Answer

In a mixture the components are just mixed physically in any ratio, keep their own properties and can be separated by physical methods like filtration. In a compound the elements are chemically combined in a fixed ratio to form a new substance with new properties, and they can be separated only by chemical methods. For example, a mixture of iron and sulphur versus the compound iron sulphide.

•Mixture: physical, any ratio, keeps properties, separated physically
•Compound: chemical, fixed ratio, new properties, separated chemically
•Example: iron+sulphur mix vs iron sulphide

Why learn this

It's how chemists tell air (mixture) from water (compound) and separate ores and medicines.

💡 Memory trick

Mixture = Mixed (any ratio, easy split); Compound = Chemically bonded (fixed, hard to split).

ChemistryTyndall effectmedium

What is the Tyndall effect, and what does it tell us about a mixture?

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What it is

The Tyndall effect is light scattering off colloid particles, revealing the beam's path.

Answer

The Tyndall effect is the scattering of a beam of light by the tiny particles of a colloid, which makes the path of the light visible. It shows that the mixture is a colloid (like milk or fog), because a true solution's particles are too small to scatter light while a colloid's particles are just the right size.

•Scattering of light by colloidal particles makes the beam visible
•Seen in colloids (milk, fog), not in true solutions
•Shows the particle size is in the colloidal range

Why learn this

It's why you see sunbeams through fog or a projector beam in a dusty room.

💡 Memory trick

If you can SEE the light beam, it's a colloid (Tyndall). Crystal-clear = true solution.

ChemistryMass percentage of a solutionmedium

20 g of salt is dissolved to make 100 g of solution. Find the mass percentage. Slide to explore.

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What it is

Mass percentage tells what fraction of a solution's mass is the dissolved solute.

Mass % = (solute ÷ solution) × 100

Mass of solute20 gMass of solution100 g

Mass percentage20.0 %

Answer

Mass percentage = (mass of solute / mass of solution) x 100 = (20 / 100) x 100 = 20%. A higher solute mass, or a smaller total solution mass, gives a higher percentage.

mass % = (mass of solute / mass of solution) x 100

•Mass % = (mass of solute / mass of solution) x 100
•Solution = solute + solvent
•Higher solute -> higher %

Why learn this

It's how the strength of solutions like saline or sugar syrup is stated.

💡 Memory trick

Mass % = (solute / solution) x 100.

ChemistryConcentration of a solutioneasy

20 g of solute is dissolved in 2 L of solution. Find the concentration. Slide to explore.

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What it is

Concentration is the amount of solute present in a given volume of solution.

Concentration = mass ÷ volume

Mass of solute20 gVolume of solution2.0 L

Concentration10.0 g/L

Answer

Concentration = mass of solute / volume of solution = 20 / 2 = 10 g/L. Dissolving the same solute in a smaller volume gives a more concentrated (stronger) solution.

concentration = mass of solute / volume of solution

•Concentration = mass of solute / volume
•Unit: g/L
•Smaller volume -> stronger solution

Why learn this

It decides how strong a medicine, drink or reagent is.

💡 Memory trick

Concentration = mass of solute / volume. Less liquid -> stronger.

ChemistryPercentage purityeasy

A 100 g sample contains 90 g of pure substance. Find its percentage purity. Slide to explore.

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What it is

Percentage purity is the fraction of a sample that is the pure substance, expressed out of a hundred.

Purity % = (pure ÷ total) × 100

Mass of pure substance90 gTotal sample mass100 g

Purity90.0 %

Answer

Percentage purity = (mass of pure substance / total mass) x 100 = (90 / 100) x 100 = 90%. The remaining 10% is impurity, so a higher percentage means a cleaner, more valuable sample.

purity % = (mass of pure substance / total mass) x 100

•Purity % = (pure mass / total mass) x 100
•The rest is impurity
•Higher purity is more valuable

Why learn this

It tells how much of a sample of gold, medicine or reagent is the real thing.

💡 Memory trick

Purity % = (pure mass / total mass) x 100.

ChemistryPure substances and mixtureseasy

Differentiate between a pure substance and a mixture.

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What it is

A pure substance is made of only one kind of particle; a mixture contains two or more substances mixed together.

Answer

A pure substance is made up of only one kind of particle and has a fixed composition and definite properties, for example an element like copper or a compound like water. A mixture contains two or more pure substances mixed together in any proportion, and its components keep their own properties and can be separated by physical methods, for example air or a sugar solution.

•Pure substance: one kind of particle, fixed composition
•Elements and compounds are pure
•Mixture: two or more substances, any ratio
•Mixtures separated by physical methods

Why learn this

It is the basic way chemists judge the composition of materials.

💡 Memory trick

Pure = single kind (element or compound). Mixture = more than one, mixed in any ratio.

ChemistryHomogeneous and heterogeneous mixtureseasy

Give one difference between homogeneous and heterogeneous mixtures with examples.

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What it is

A homogeneous mixture has a uniform composition; a heterogeneous mixture has a non-uniform composition with visible parts.

Answer

A homogeneous mixture has a uniform composition throughout and no visible boundaries between its components, for example a solution of salt in water or air. A heterogeneous mixture has a non-uniform composition, and its components can be seen and physically separated, for example a mixture of sand and iron filings or oil and water.

•Homogeneous: uniform, no visible parts (salt water)
•Heterogeneous: non-uniform, visible parts (sand + iron)
•Solutions are homogeneous
•Suspensions are heterogeneous

Why learn this

It classifies mixtures by how evenly their components are spread.

💡 Memory trick

Homogeneous = uniform (salt water). Heterogeneous = uneven, parts visible (sand + salt).

ChemistrySolution, colloid and suspensionmedium

Differentiate between a true solution, a colloid and a suspension.

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What it is

Mixtures are classified by particle size: true solutions (smallest), colloids (medium, Tyndall effect) and suspensions (largest, settle down).

Answer

A true solution is a homogeneous mixture with very small particles (less than 1 nm) that do not settle or scatter light, for example salt in water. A colloid is a heterogeneous mixture with medium-sized particles that do not settle but scatter a beam of light, showing the Tyndall effect, for example milk. A suspension is a heterogeneous mixture with large particles that settle down on standing and can be filtered, for example chalk in water.

•Solution: tiny particles, no scattering, no settling
•Colloid: shows Tyndall effect (milk)
•Suspension: large particles, settle down (chalk + water)
•Classified by particle size

Why learn this

It explains why milk (colloid) scatters light and muddy water (suspension) settles.

💡 Memory trick

Solution = clear, no settling; Colloid = shows Tyndall effect; Suspension = settles down.

ChemistryConcentration of a solutionmedium

A solution contains 20 g of salt in 80 g of water. Find the mass percentage of the solute.

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What it is

The concentration of a solution is the amount of solute present in a given amount of solution, often as mass percentage.

Answer

Mass of solution = mass of solute + mass of solvent = 20 + 80 = 100 g. Mass percentage of solute = (mass of solute / mass of solution) x 100 = (20/100) x 100 = 20%. So the solution is 20% salt by mass.

Mass % of solute = (mass of solute / mass of solution) x 100

•Mass of solution = solute + solvent = 100 g
•Mass % = (solute/solution) x 100
•(20/100) x 100 = 20%
•Concentration = amount of solute per solution

Why learn this

It tells how strong a solution is, important in medicine and industry.

💡 Memory trick

Mass % of solute = (mass of solute / mass of solution) x 100.

ChemistrySeparation techniquesmedium

Name the technique used to separate: (a) salt from sea water, (b) two miscible liquids, (c) cream from milk.

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What it is

Mixtures are separated by physical methods chosen according to the properties of their components.

Answer

Salt from sea water is separated by evaporation, where the water evaporates leaving the salt behind. Two miscible liquids with different boiling points are separated by fractional distillation. Cream from milk is separated by centrifugation, in which rapid spinning throws the denser and lighter parts apart. Other methods include filtration, sublimation and chromatography.

•Evaporation: salt from sea water
•Fractional distillation: miscible liquids
•Centrifugation: cream from milk
•Method depends on the property that differs

Why learn this

It lets us obtain pure substances, like getting salt from sea water.

💡 Memory trick

Match the method to the property: boiling point (distillation), density (centrifuge), solubility (crystallisation).

ChemistryElements and compoundsmedium

How is a compound different from a mixture?

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What it is

An element contains only one kind of atom; a compound is formed when elements combine chemically in a fixed ratio.

Answer

A compound is a pure substance formed when two or more elements combine chemically in a fixed proportion by mass, and it has properties entirely different from its constituent elements; its components can be separated only by chemical methods. A mixture is formed by simply mixing substances in any ratio, its components keep their own properties, and they can be separated by physical methods. For example, water is a compound of hydrogen and oxygen, while air is a mixture of gases.

•Compound: fixed ratio, new properties (water)
•Mixture: any ratio, keeps properties (air)
•Compound separated only chemically
•Mixture separated physically

Why learn this

It shows how the huge variety of substances is built from about a hundred elements.

💡 Memory trick

Compound has NEW properties and a FIXED ratio; a mixture keeps old properties and any ratio.

Atoms and Molecules14 questions

ChemistryLaw of conservation of massmedium

State the law of conservation of mass with an example.

Reveal answer▾

What it is

In any chemical reaction the total mass stays the same - atoms are only rearranged, never lost.

Answer

The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction; the total mass of the reactants equals the total mass of the products. For example, when 12 g of carbon burns completely in 32 g of oxygen, exactly 44 g of carbon dioxide is formed (12 + 32 = 44).

mass of reactants = mass of products

•Mass is neither created nor destroyed
•Total mass of reactants = total mass of products
•Example: 12 g C + 32 g O2 -> 44 g CO2

Why learn this

It's why every chemical equation must be BALANCED - the foundation of all of chemistry.

💡 Memory trick

Atoms in = atoms out. Nothing vanishes, it just rearranges.

ChemistryMole conceptmedium

How many moles are there in 36 g of water (molar mass 18 g/mol)? Slide to explore.

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What it is

The mole links the mass of a substance to the number of particles it contains.

Moles = mass ÷ molar mass

Given mass36 gMolar mass18 g/mol

Amount2.00 mol

Answer

Number of moles = given mass / molar mass = 36 / 18 = 2 mol. The molar mass is the mass of one mole of the substance, so dividing the given mass by it gives the number of moles.

n = mass / molar mass

•Moles = given mass / molar mass
•Molar mass = mass of 1 mole (g/mol)
•18 g of water = 1 mole

Why learn this

Chemists weigh substances but react them in whole numbers of particles - the mole bridges the two.

💡 Memory trick

Moles = given mass / molar mass. 18 g of water = 1 mole.

ChemistryAvogadro's numbermedium

How many molecules are there in 1 mole? And in 2 moles? Slide to explore.

Reveal answer▾

What it is

One mole of any substance contains a fixed, huge number of particles - Avogadro's number.

Particles = moles × 6.022 × 10²³

Amount1.0 mol

Particles6.02 × 10²³

Answer

Number of particles = moles x 6.022 x 10^23. So 1 mole contains 6.022 x 10^23 particles, and 2 moles contain 1.2044 x 10^24 particles. This count, 6.022 x 10^23 per mole, is Avogadro's number and is the same for every substance.

N = n x 6.022 x 10^23

•Particles = moles x 6.022 x 10^23
•Avogadro's number = 6.022 x 10^23 per mole
•Same count for every substance

Why learn this

It lets us count atoms and molecules by simply weighing a sample.

💡 Memory trick

Particles = moles x 6.022 x 10^23, the same count for every substance.

ChemistryMoles from number of particlesmedium

How many moles are 6.022 x 10^23 particles? And 1.2 x 10^24? Slide to explore.

Reveal answer▾

What it is

The number of moles equals the number of particles divided by Avogadro's number.

Moles = particles ÷ (6.022 × 10²³)

Particles (×10²³)6

Amount1.00 mol

Answer

Moles = number of particles / (6.022 x 10^23). So 6.022 x 10^23 particles = 1 mole, and 1.2044 x 10^24 particles = 2 moles. Avogadro's number is simply the number of particles in one mole.

n = N / (6.022 x 10^23)

•Moles = particles / (6.022 x 10^23)
•6.022 x 10^23 particles = 1 mole
•Reverse of particles = moles x Avogadro's number

Why learn this

It lets us convert a count of atoms or molecules into moles for calculations.

💡 Memory trick

Moles = particles / (6.022 x 10^23).

ChemistryMass from molesmedium

What is the mass of 2 moles of water (molar mass 18 g/mol)? Slide to explore.

Reveal answer▾

What it is

The mass of a sample is the number of moles multiplied by the molar mass.

Mass = moles × molar mass

Amount2.0 molMolar mass18 g/mol

Mass36.0 g

Answer

Mass = moles x molar mass = 2 x 18 = 36 g. This is simply the mole formula rearranged: since moles = mass / molar mass, mass = moles x molar mass.

mass = moles x molar mass

•Mass = moles x molar mass
•Rearranged from moles = mass / molar mass
•2 mol of water (18) = 36 g

Why learn this

It lets chemists weigh out an exact number of moles for a reaction.

💡 Memory trick

Mass = moles x molar mass - the reverse of moles = mass / molar mass.

ChemistryWriting chemical formulae (valency)medium

How do you write the formula of calcium chloride? Pick the ions to criss-cross their valencies.

Reveal answer▾

What it is

A chemical formula is written by criss-crossing the valencies of the cation and anion, then simplifying.

Build a formula (valency criss-cross)

Cation (metal)

Anion (non-metal / radical)

CaCl₂

Calcium Chloride

Criss-cross the valencies: the cation's valency (2) becomes the anion's subscript and the anion's valency (1) becomes the cation's, then simplify. Brackets group a radical that repeats.

Answer

Calcium is Ca with valency 2 and chloride is Cl with valency 1. Criss-crossing, the calcium valency (2) becomes the chloride subscript and the chloride valency (1) becomes the calcium subscript, giving CaCl2. For a radical that repeats, such as (NO3), we put it in brackets: calcium nitrate is Ca(NO3)2.

criss-cross valencies, then simplify

•Criss-cross the valencies into subscripts
•Simplify the subscripts where possible
•Ca(2) + Cl(1) -> CaCl2; Al(3) + O(2) -> Al2O3
•Use brackets for a repeating radical

Why learn this

It lets you write the correct formula of any ionic compound from the valencies.

💡 Memory trick

Criss-cross: each ion's valency becomes the other's subscript, then simplify. Brackets group a radical.

ChemistrySymbols of elementseasy

Can you match each element to its correct symbol? Tap the right symbol to score.

Reveal answer▾

What it is

Each element has a one or two letter symbol, often from its English or Latin name.

Match the symbol1 / 8

Symbol for Hydrogen?

Tap the correct symbol.

Answer

Element symbols use a capital first letter and, if needed, a small second letter: Hydrogen is H, Helium He, Carbon C, Nitrogen N, Oxygen O. Some come from Latin names - Sodium is Na (natrium), Iron is Fe (ferrum) and Gold is Au (aurum). Learning them is essential because every chemical formula is written using these symbols.

•First letter capital, second letter small
•H, He, C, N, O for common non-metals
•Latin names: Na (sodium), Fe (iron), Au (gold)

Why learn this

Symbols are the alphabet of chemistry - every formula and equation is built from them.

💡 Memory trick

First letter capital, second small: Sodium = Na (from natrium), Iron = Fe (ferrum), Gold = Au (aurum).

ChemistryLaw of conservation of massmedium

State the law of conservation of mass with an example.

Reveal answer▾

What it is

In a chemical reaction, mass can neither be created nor destroyed; the total mass of reactants equals the total mass of products.

Answer

The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction; the total mass of the products is always equal to the total mass of the reactants. For example, when carbon burns in oxygen, 12 g of carbon combines with 32 g of oxygen to form exactly 44 g of carbon dioxide (12 + 32 = 44). This is why chemical equations are balanced.

Mass of reactants = mass of products

•Mass of reactants = mass of products
•Mass is neither created nor destroyed
•Atoms are only rearranged
•Basis for balancing equations

Why learn this

It is why chemical equations must be balanced.

💡 Memory trick

Mass in = mass out. Atoms are only rearranged, never lost.

ChemistryLaw of constant proportionsmedium

State the law of constant proportions with an example.

Reveal answer▾

What it is

A chemical compound always contains the same elements combined in the same fixed proportion by mass.

Answer

The law of constant (definite) proportions states that a pure chemical compound always contains the same elements combined together in the same fixed proportion by mass, whatever its source or method of preparation. For example, in pure water the ratio of the mass of hydrogen to the mass of oxygen is always 1:8, whether the water comes from a river, rain or a laboratory.

Fixed mass ratio of elements in a compound

•Elements combine in a fixed mass ratio
•Ratio is the same from any source
•Water: hydrogen to oxygen = 1:8
•Given by Proust

Why learn this

It is why water is always H2O with hydrogen and oxygen in a fixed 1:8 mass ratio.

💡 Memory trick

Same compound = same fixed ratio, no matter the source or amount.

ChemistryAtoms, molecules and atomicitymedium

Define a molecule and atomicity. What is the atomicity of oxygen and ozone?

Reveal answer▾

What it is

An atom is the smallest particle of an element; a molecule is a group of atoms bonded together; atomicity is the number of atoms in a molecule.

Answer

A molecule is the smallest particle of an element or compound that can exist independently and shows all the properties of that substance. Atomicity is the number of atoms present in one molecule of an element. The atomicity of oxygen (O2) is 2, so it is diatomic, and the atomicity of ozone (O3) is 3, so it is triatomic. Noble gases such as helium are monatomic (atomicity 1).

Atomicity = number of atoms in one molecule

•Atom: smallest particle of an element
•Molecule: group of atoms that exists independently
•Atomicity: number of atoms in a molecule
•O2 diatomic, O3 triatomic, He monatomic

Why learn this

It explains how oxygen exists as O2 and ozone as O3.

💡 Memory trick

Atomicity of O2 = 2 (diatomic); of O3 = 3; of noble gases = 1 (monatomic).

ChemistryIons and valencymedium

What is an ion? Differentiate between a cation and an anion.

Reveal answer▾

What it is

An ion is a charged atom or group of atoms; valency is the combining capacity of an element.

Answer

An ion is an atom or group of atoms that carries a net electric charge because it has lost or gained electrons. A cation is a positively charged ion formed when an atom loses one or more electrons, for example the sodium ion Na+. An anion is a negatively charged ion formed when an atom gains one or more electrons, for example the chloride ion Cl-. The valency is the number of electrons lost, gained or shared.

•Ion = charged atom or group of atoms
•Cation: positive, loses electrons (Na+)
•Anion: negative, gains electrons (Cl-)
•Valency = combining capacity

Why learn this

Ions and valency let us write correct formulae for ionic compounds like NaCl.

💡 Memory trick

Lose electrons -> positive cation; gain electrons -> negative anion.

ChemistryWriting chemical formulaemedium

Write the chemical formula of aluminium oxide using the criss-cross method.

Reveal answer▾

What it is

A chemical formula is written by criss-crossing the valencies of the combining ions.

Answer

Aluminium has a valency of 3 (Al with charge 3+) and oxygen has a valency of 2 (O with charge 2-). Using the criss-cross method, the valency of aluminium (3) becomes the subscript of oxygen and the valency of oxygen (2) becomes the subscript of aluminium, giving Al2O3. So the formula of aluminium oxide is Al2O3.

Criss-cross valencies to write the formula

•Write symbols with their valencies
•Criss-cross the valency numbers as subscripts
•Al valency 3, O valency 2
•Formula: Al2O3

Why learn this

It gives the exact composition of a compound in shorthand.

💡 Memory trick

Criss-cross the valencies: the valency of one becomes the subscript of the other.

ChemistryMole concepthard

How many molecules are present in 2 moles of water? What is their mass? (Molar mass of water = 18 g/mol.)

Reveal answer▾

What it is

One mole of any substance contains Avogadro's number (6.022 x 10^23) of particles and has a mass equal to its molar mass.

Answer

One mole contains Avogadro's number of particles, 6.022 x 10^23. So 2 moles of water contain 2 x 6.022 x 10^23 = 1.2044 x 10^24 molecules. The mass = number of moles x molar mass = 2 x 18 = 36 g. So 2 moles of water contain about 1.2 x 10^24 molecules and have a mass of 36 g.

Number of particles = moles x 6.022 x 10^23 ; mass = moles x molar mass

•1 mole = 6.022 x 10^23 particles
•Molecules = moles x Avogadro number
•2 x 6.022 x 10^23 = 1.2 x 10^24
•Mass = moles x molar mass = 36 g

Why learn this

The mole links the number of particles to a weighable mass in the lab.

💡 Memory trick

1 mole = 6.022 x 10^23 particles = molar mass in grams.

ChemistryMolecular massmedium

Calculate the molecular mass of sulphuric acid, H2SO4. (H = 1, S = 32, O = 16.)

Reveal answer▾

What it is

The molecular mass of a substance is the sum of the atomic masses of all the atoms in its molecule.

Answer

The formula H2SO4 contains 2 hydrogen atoms, 1 sulphur atom and 4 oxygen atoms. Molecular mass = (2 x 1) + (1 x 32) + (4 x 16) = 2 + 32 + 64 = 98 u. So the molecular mass of sulphuric acid is 98 atomic mass units (its molar mass is 98 g/mol).

Molecular mass = sum of atomic masses of all atoms

•Add atomic masses of all atoms
•H2SO4: 2(1) + 32 + 4(16)
•= 2 + 32 + 64 = 98 u
•Molar mass = 98 g/mol

Why learn this

It is needed to convert between mass and moles in every calculation.

💡 Memory trick

Add up the atomic masses of every atom in the formula.

Structure of the Atom14 questions

ChemistryAtomic number and mass numbereasy

Define the atomic number and the mass number of an atom.

Reveal answer▾

What it is

Atomic number = number of protons (the element's identity); mass number = protons + neutrons.

Protons = 6Neutrons = 6

Z = 6 · A = 6 + 6 = 12 · Carbon (C)

Build an atom — protons set the element (Z); protons + neutrons give the mass number (A)

Answer

The atomic number (Z) is the number of protons in the nucleus of an atom, which also equals the number of electrons in a neutral atom; it decides the identity of the element. The mass number (A) is the total number of protons and neutrons in the nucleus. So the number of neutrons = A - Z.

A = Z + number of neutrons

•Atomic number Z = number of protons (= electrons if neutral)
•Mass number A = protons + neutrons
•Neutrons = A - Z
•Z identifies the element

Why learn this

These two numbers decide every element in the periodic table and its isotopes.

💡 Memory trick

Z = protons (Zee identity); A = All heavy bits (protons + neutrons). Neutrons = A - Z.

ChemistryIsotopesmedium

What are isotopes? Give one example.

Reveal answer▾

What it is

Isotopes are atoms of the same element with the same protons but different numbers of neutrons.

Answer

Isotopes are atoms of the same element that have the same atomic number (the same number of protons) but different mass numbers (different numbers of neutrons). Because they have the same number of protons and electrons, they show the same chemical properties. For example, carbon-12 and carbon-14 are isotopes of carbon.

Same Z, different A

•Same element, same protons (same atomic number)
•Different neutrons (different mass number)
•Same chemical properties
•Example: carbon-12 and carbon-14

Why learn this

Isotopes are used in cancer treatment, carbon dating and nuclear energy.

💡 Memory trick

Same element (same protons), different mass (different neutrons). ISO = same place in the table.

ChemistryElectronic configuration and valencymedium

How do you write the electronic configuration and find the valency of an element? Slide the atomic number from H to Ca to explore.

Reveal answer▾

What it is

The atomic number fixes how many electrons an atom has; filling shells by the 2, 8, 8 rule gives its electronic configuration, valence electrons and valency.

Atomic number, configuration & valency

Carbon

Atomic number Z = 6

6 protons, 6 electrons

Atomic number (Z)6

Configuration: 2, 4 (K, L)

Valence electrons: 4

Valence electrons are 1–4, so valency = 4.

Valency = 4

Answer

Start with the atomic number Z = number of protons = number of electrons in a neutral atom. Fill the shells K, L, M, N using the 2, 8, 8, 2 pattern (the Bohr-Bury scheme) to get the electronic configuration. The electrons in the outermost shell are the valence electrons. Then: if there are 1-4 valence electrons the valency equals that number; if there are 5-7, valency = 8 - (valence electrons); and if the outer shell is complete (2 for helium, 8 for the other noble gases) the valency is 0. Example: carbon (Z = 6) is 2, 4, so 4 valence electrons and valency 4. Helium (Z = 2) has a full K shell of 2, so its valency is 0.

valency: 1-4 -> n ; 5-7 -> 8 - n ; full shell -> 0

•Z = protons = electrons (neutral atom)
•Fill shells by 2, 8, 8, 2
•Valence electrons = outermost shell
•1-4 -> valency = that number; 5-7 -> 8 - electrons; full shell -> 0
•Helium's outer shell (2) is full, so valency = 0

Why learn this

Valency decides how atoms combine, so it is the key to every chemical formula.

💡 Memory trick

Fill shells 2, 8, 8. Valence 1-4 -> valency = that number; 5-7 -> 8 minus it; full shell -> 0.

ChemistryMass numbereasy

An atom has 6 protons and 6 neutrons. Find its mass number. Slide to explore.

Reveal answer▾

What it is

The mass number of an atom is the total count of protons and neutrons in its nucleus.

Mass number = protons + neutrons

Protons (Z)6Neutrons6

Mass number (A)12

Answer

Mass number A = protons + neutrons = 6 + 6 = 12 (this is carbon-12). The number of protons is the atomic number and fixes which element it is; changing only the neutrons gives a different isotope of the same element.

A = protons + neutrons

•Mass number A = protons + neutrons
•Protons = atomic number (Z)
•Same Z, different neutrons -> isotopes

Why learn this

It distinguishes isotopes and gives the atom almost all of its mass.

💡 Memory trick

A = p + n. Protons name the element; neutrons add mass.

ChemistryElectron dot structuremedium

How do you draw the electron dot structure of an element? Slide the atomic number to explore.

Reveal answer▾

What it is

An electron dot (Lewis) structure shows only the valence electrons of an atom as dots around its symbol.

Electron dot (Lewis) structure

Atomic number (Z)6 · Carbon

Configuration: 2, 4

Valence electrons (dots): 4

Only the outermost electrons are drawn - these are the ones that form bonds.

Answer

First find the number of valence electrons (the electrons in the outermost shell) from the electronic configuration. Then write the element's symbol and place that many dots around it, one for each valence electron. For example, carbon (2, 4) has 4 valence electrons, so 4 dots; oxygen (2, 6) has 6 dots; sodium (2, 8, 1) has just 1. These outer electrons are the ones involved in chemical bonding.

dots = number of valence electrons

•Show only the valence (outer-shell) electrons
•One dot per valence electron, around the symbol
•Carbon -> 4 dots, oxygen -> 6 dots, sodium -> 1 dot
•These electrons form chemical bonds

Why learn this

Those outer electrons are the ones that form bonds, so dot structures explain how atoms join.

💡 Memory trick

Draw the symbol, then one dot per valence electron around it (up to 8).

ChemistryAverage atomic mass of isotopesmedium

Chlorine is 75% mass-35 and 25% mass-37. Find its average atomic mass. Slide to explore.

Reveal answer▾

What it is

The average atomic mass is the weighted mean of the masses of an element's isotopes, using their percentage abundances.

Average mass = (m₁×a₁ + m₂×a₂) ÷ 100

Isotope 1 mass35 uIsotope 2 mass37 uIsotope 1 abundance75 %

Average atomic mass35.50 u

Answer

Average atomic mass = (35 x 75 + 37 x 25) / 100 = (2625 + 925) / 100 = 3550 / 100 = 35.5 u. It is a weighted average, so the more abundant isotope pulls the value closer to its own mass.

average = (m1 x a1 + m2 x a2) / 100

•Weighted mean of isotope masses
•Average = (m1 x a1 + m2 x a2) / 100
•Explains non-whole atomic masses like 35.5

Why learn this

It's why atomic masses like chlorine's 35.5 are not whole numbers.

💡 Memory trick

Average = (m1 x a1 + m2 x a2) / 100, with a1 + a2 = 100.

ChemistryNumber of neutronseasy

An atom has mass number 12 and atomic number 6. How many neutrons does it have? Slide to explore.

Reveal answer▾

What it is

The number of neutrons in an atom is its mass number minus its atomic number.

Neutrons = mass number − atomic number

Mass number A12Atomic number Z6

Neutrons6

Answer

Neutrons = mass number - atomic number = 12 - 6 = 6. The atomic number gives the protons, so subtracting it from the mass number (protons + neutrons) leaves the number of neutrons.

neutrons = A - Z

•Neutrons = A - Z
•A = mass number, Z = atomic number
•Carbon-12 has 6 neutrons

Why learn this

It's how we count the neutrons that distinguish one isotope from another.

💡 Memory trick

Neutrons = A - Z (mass number minus protons).

ChemistrySubatomic particleseasy

Name the three subatomic particles with their charge, mass and discoverer.

Reveal answer▾

What it is

An atom is made of protons and neutrons in the nucleus, with electrons in shells around it.

Answer

The three subatomic particles are: the electron, which has a negative charge and negligible mass and was discovered by J. J. Thomson; the proton, which has a positive charge and a mass of about 1 u and was discovered by Goldstein; and the neutron, which has no charge and a mass of about 1 u and was discovered by James Chadwick. Protons and neutrons are in the nucleus, and electrons revolve around it.

•Electron: negative, negligible mass (Thomson)
•Proton: positive, 1 u (Goldstein)
•Neutron: neutral, 1 u (Chadwick)
•Protons + neutrons in nucleus; electrons outside

Why learn this

It is the basis for understanding chemical behaviour and the periodic table.

💡 Memory trick

Proton (+, discovered by Goldstein), Electron (-, Thomson), Neutron (0, Chadwick).

ChemistryThomson and Rutherford modelsmedium

Describe Rutherford's model of the atom and how it improved on Thomson's model.

Reveal answer▾

What it is

Thomson pictured the atom as positive matter with embedded electrons; Rutherford's experiment showed a small, dense, positive nucleus.

Answer

Thomson's model described the atom as a sphere of positive charge with negatively charged electrons embedded in it, like plums in a pudding. Rutherford's alpha-particle scattering experiment showed that most of the atom is empty space, that there is a very small, dense, positively charged centre called the nucleus where the mass is concentrated, and that electrons revolve around this nucleus. This improved on Thomson's model by introducing the nucleus.

•Thomson: positive sphere with embedded electrons
•Rutherford: alpha scattering experiment
•Small, dense, positive nucleus
•Electrons revolve around the nucleus

Why learn this

Rutherford's model corrected Thomson's and introduced the nucleus.

💡 Memory trick

Thomson = 'plum pudding'. Rutherford = alpha-scattering revealed the tiny nucleus.

ChemistryBohr's model and energy shellsmedium

State the main postulates of Bohr's model of the atom.

Reveal answer▾

What it is

Bohr proposed that electrons revolve around the nucleus only in certain fixed energy shells without radiating energy.

Answer

According to Bohr's model, electrons revolve around the nucleus only in certain permitted circular paths called energy levels or shells, which are named K, L, M, N and so on. While revolving in these shells, an electron does not lose or gain energy, so the atom is stable. Energy is absorbed or emitted only when an electron jumps from one shell to another. This model explained the stability of the atom.

Shells K, L, M, N for n = 1, 2, 3, 4

•Electrons revolve in fixed shells (K, L, M, N)
•No energy lost while in a shell (stable atom)
•Energy change only on jumping shells
•Improved atomic stability over Rutherford

Why learn this

It explained the stability of atoms, which Rutherford's model could not.

💡 Memory trick

Shells are named K, L, M, N (n = 1, 2, 3, 4) - fixed energy 'orbits'.

ChemistryAtomic number and mass numbereasy

An atom has 17 protons and 18 neutrons. Find its atomic number and mass number.

Reveal answer▾

What it is

Atomic number is the number of protons; mass number is the sum of protons and neutrons.

Answer

The atomic number (Z) equals the number of protons, so Z = 17. The mass number (A) equals the number of protons plus the number of neutrons, so A = 17 + 18 = 35. This atom is chlorine, written as Cl with mass number 35.

A = Z + number of neutrons

•Atomic number Z = protons = 17
•Mass number A = protons + neutrons
•A = 17 + 18 = 35
•Element is chlorine (Cl-35)

Why learn this

These numbers identify an element and its isotopes.

💡 Memory trick

Z = protons; A = protons + neutrons; neutrons = A - Z.

ChemistryIsotopes and isobarsmedium

Differentiate between isotopes and isobars with an example each.

Reveal answer▾

What it is

Isotopes have the same atomic number but different mass numbers; isobars have the same mass number but different atomic numbers.

Answer

Isotopes are atoms of the same element that have the same atomic number but different mass numbers because they have different numbers of neutrons, for example carbon-12 and carbon-14. Isobars are atoms of different elements that have different atomic numbers but the same mass number, for example calcium-40 and argon-40. Isotopes have identical chemical properties, while isobars do not.

•Isotopes: same Z, different A (C-12, C-14)
•Isobars: same A, different Z (Ca-40, Ar-40)
•Isotopes differ in neutrons
•Isotopes have the same chemical properties

Why learn this

Isotopes explain fractional atomic masses and are used in medicine and energy.

💡 Memory trick

Iso-TOP-es: same TOP (protons/atomic number). Iso-BAR-s: same mass number (A).

ChemistryElectronic configurationmedium

Write the electronic configuration of magnesium (atomic number 12) and state its valency.

Reveal answer▾

What it is

Electrons fill shells according to the rule that a shell can hold a maximum of 2n^2 electrons, filling inner shells first.

Answer

Magnesium has 12 electrons. Using the rule that each shell holds a maximum of 2n^2 electrons and inner shells fill first: K shell holds 2, L shell holds 8, and the remaining 2 go into the M shell. So the electronic configuration is 2, 8, 2. Since it has 2 electrons in the outermost shell, magnesium loses them easily, so its valency is 2.

Maximum electrons in shell n = 2n^2

•Maximum electrons in a shell = 2n^2
•K=2, L=8, then M
•Magnesium (12): 2, 8, 2
•Valency = 2

Why learn this

The arrangement of electrons decides an element's chemical behaviour and valency.

💡 Memory trick

Max electrons per shell = 2n^2: K=2, L=8, M=18. Fill from the innermost outward.

ChemistryValency from electronic configurationmedium

How is valency determined from the electronic configuration? Find the valency of chlorine (2, 8, 7).

Reveal answer▾

What it is

Valency is found from the number of valence (outermost) electrons: it equals that number if it is 1 to 4, or 8 minus that number if it is 4 to 8.

Answer

Valency is the combining capacity of an atom, determined by the number of electrons in its outermost shell (valence electrons). If the outermost shell has 1 to 4 electrons, the valency equals the number of valence electrons; if it has 5 to 8 electrons, the valency equals 8 minus the number of valence electrons, because the atom tends to complete its octet. Chlorine has the configuration 2, 8, 7, so it has 7 valence electrons and its valency is 8 - 7 = 1.

Valency = valence electrons or (8 - valence electrons)

•Valency from valence (outermost) electrons
•1 to 4 electrons: valency = number of electrons
•5 to 8 electrons: valency = 8 - number
•Chlorine (2,8,7): valency = 8 - 7 = 1

Why learn this

It lets us predict how atoms combine without memorising every valency.

💡 Memory trick

Outer electrons 1-4 -> valency = that number. Outer electrons 5-8 -> valency = 8 minus it.

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