For Class 8, 9 & 10
Master the basics - and everything after gets easier
Concept-first questions with clear model answers in Physics, Chemistry, Maths and Biology, all NCERT-aligned. Start early, build the habit, and walk into your boards, NEET and JEE already ahead.
Interactive lessons
learn by playingDrag, slide and build - watch each concept come alive, then reveal the answer.
200 interactive lessons
Ohm's law
Class 10 Physics
Slide V & R, watch the bulb glow
Open →pH scale
Class 10 Chemistry
Slide across acids and bases
Open →Atomic number and mass number
Class 9 Chemistry
Add protons & neutrons, build shells
Open →Laws of reflection
Class 8 Physics
Change the angle, watch it bounce
Open →Volume of a sphere
Class 9 Maths
Grow the radius, see the volume
Open →Area of a trapezium
Class 8 Maths
Drag the sides, read the area
Open →Power of a lens
Class 10 Physics
Move the object, trace the rays
Open →Food chain and energy flow
Class 10 Biology
Follow the energy as it flows
Open →Speed
Class 8 Physics
Slide distance & time, watch the speed
Open →Density
Class 9 Physics
Pack mass into volume, float or sink
Open →Work done
Class 9 Physics
Push harder or farther, watch work grow
Open →Kinetic energy
Class 9 Physics
Speed it up - energy grows with the square
Open →Power of a lens
Class 10 Physics
Shorten the focal length, boost the power
Open →Mole concept
Class 9 Chemistry
Weigh out grams, count the moles
Open →Avogadro's number
Class 9 Chemistry
Add moles, count the particles
Open →Microscope magnification
Class 8 Biology
Grow the image, read the magnification
Open →Population density
Class 10 Biology
Add individuals, shrink the land, see crowding
Open →Simple interest
Class 8 Maths
Slide money, rate & time, watch interest
Open →Pythagoras theorem
Class 9 Maths
Stretch the two sides, get the hypotenuse
Open →Probability of an event
Class 10 Maths
Change the outcomes, watch the odds
Open →Newton's second law
Class 9 Physics
Push a mass, pick an acceleration
Open →Momentum
Class 9 Physics
Slide mass & velocity, build momentum
Open →Pressure
Class 8 Physics
Shrink the area, feel the pressure rise
Open →Weight
Class 9 Physics
Change the planet's gravity, watch your weight
Open →Refractive index
Class 10 Physics
Slow light in the medium, raise the index
Open →Resistors in series
Class 10 Physics
Add two resistors in a line
Open →Mass percentage of a solution
Class 9 Chemistry
Dissolve solute, read the strength
Open →Concentration of a solution
Class 9 Chemistry
Pack solute into less liquid
Open →Population change
Class 10 Biology
Balance births against deaths
Open →Compound microscope
Class 8 Biology
Combine eyepiece & objective lenses
Open →Area of a circle
Class 8 Maths
Grow the radius, watch the area square
Open →Volume of a cuboid
Class 8 Maths
Stretch length, breadth & height
Open →Electronic configuration and valency
Class 9 Chemistry
Slide the atomic number, build the atom
Open →Homologous series (alkanes)
Class 10 Chemistry
Add carbons, name the compound
Open →Mass number
Class 9 Chemistry
Add protons & neutrons, get the mass number
Open →Power
Class 9 Physics
More work in less time = more power
Open →Potential energy
Class 9 Physics
Lift a mass higher, store energy
Open →Wave speed
Class 9 Physics
Tune frequency & wavelength, set the speed
Open →Electric current
Class 10 Physics
Push charge per second, get the current
Open →Percentage
Class 8 Maths
Compare part to whole as a %
Open →Electron dot structure
Class 9 Chemistry
Draw valence electrons as dots
Open →Acceleration
Class 9 Physics
Speed up over time, find acceleration
Open →Distance, speed and time
Class 8 Physics
Set speed & time, cover the distance
Open →Frequency and time period
Class 9 Physics
Shorten the period, raise the frequency
Open →Heating effect of current
Class 10 Physics
Raise the current, watch heating soar
Open →Area of a triangle
Class 8 Maths
Set base & height, halve the rectangle
Open →Area of a rectangle
Class 8 Maths
Set length & breadth, fill the area
Open →Mean (average)
Class 9 Maths
Share the total equally across items
Open →Discount
Class 8 Maths
Slide price & % off, see the saving
Open →Heart rate
Class 10 Biology
Set heart rate & time, count the beats
Open →Resistors in parallel
Class 10 Physics
Wire two resistors side by side
Open →Electric charge
Class 10 Physics
Flow current over time, collect charge
Open →Electrical energy and units
Class 10 Physics
Run appliances, add up the units
Open →Kelvin temperature scale
Class 9 Chemistry
Slide Celsius, read the Kelvin
Open →Moles from number of particles
Class 9 Chemistry
Divide particles by Avogadro's number
Open →Ten percent law
Class 10 Biology
See 10% of energy reach the next level
Open →Area of a square
Class 8 Maths
Grow the side, square the area
Open →Volume of a cube
Class 8 Maths
Grow the edge, cube the volume
Open →Circumference of a circle
Class 8 Maths
Grow the radius, roll out the rim
Open →Surface area of a cube
Class 9 Maths
Grow the edge, cover six faces
Open →Potential difference
Class 10 Physics
Share work across charge, get volts
Open →Resistance from Ohm's law
Class 10 Physics
Divide voltage by current, get resistance
Open →Echo and SONAR
Class 9 Physics
Time the echo, find the distance
Open →Mass from moles
Class 9 Chemistry
Multiply moles by molar mass
Open →Breathing rate
Class 10 Biology
Set breathing rate & time
Open →Volume of a cylinder
Class 10 Maths
Set radius & height, fill the can
Open →Compound interest
Class 8 Maths
Compound money over years
Open →Profit and loss percentage
Class 8 Maths
Set cost & selling price, see profit %
Open →Perimeter of a rectangle
Class 8 Maths
Set length & breadth, walk the border
Open →Surface area of a sphere
Class 10 Maths
Grow the radius, wrap the ball
Open →Time period
Class 9 Physics
Raise the frequency, shrink the period
Open →Relative velocity
Class 9 Physics
Two objects approach - add their speeds
Open →Average velocity
Class 9 Physics
Average the start and end speeds
Open →Equations of motion (v = u + at)
Class 9 Physics
Accelerate from u for a time t
Open →Kelvin to Celsius
Class 9 Chemistry
Slide Kelvin, read the Celsius
Open →Population growth rate
Class 10 Biology
Balance births vs deaths per population
Open →Perimeter of a square
Class 8 Maths
Grow the side, walk four edges
Open →Perimeter of a triangle
Class 8 Maths
Add the three sides
Open →Area of a parallelogram
Class 8 Maths
Set base & height, slide the shape
Open →Area of a rhombus
Class 8 Maths
Set the two diagonals
Open →Equations of motion (distance)
Class 9 Physics
Start, accelerate, cover ground
Open →Joule's law of heating
Class 10 Physics
Raise current, resistance or time
Open →Electric power (P = VI)
Class 10 Physics
Multiply voltage by current
Open →Average atomic mass of isotopes
Class 9 Chemistry
Mix two isotopes by abundance
Open →Seed germination percentage
Class 9 Biology
Count sprouted seeds out of the total
Open →Volume of a cone
Class 9 Maths
Set radius & height, fill the cone
Open →Surface area of a cylinder
Class 9 Maths
Wrap the side and both ends
Open →Surface area of a cuboid
Class 9 Maths
Cover all six rectangular faces
Open →nth term of an AP
Class 10 Maths
Step from the first term by d
Open →Sum of an AP
Class 10 Maths
Add up the first n terms
Open →Focal length of a mirror
Class 10 Physics
Halve the radius to find the focus
Open →Speed of light in a medium
Class 10 Physics
Raise the index, slow the light
Open →Percentage purity
Class 9 Chemistry
Weigh the pure part of a sample
Open →Slope of a line
Class 10 Maths
Rise over run gives the steepness
Open →Percentage change
Class 8 Maths
Compare a new value to the old
Open →Volume of a hemisphere
Class 9 Maths
Grow the radius of half a ball
Open →Area of a sector
Class 10 Maths
Cut a slice of angle from a circle
Open →Length of an arc
Class 10 Maths
Measure the curved edge of a slice
Open →Slant height of a cone
Class 9 Maths
Combine radius & height for the slant
Open →Unit conversion (km/h to m/s)
Class 9 Physics
Convert km/h into m/s
Open →Equations of motion (v^2 = u^2 + 2as)
Class 9 Physics
Accelerate over a distance, find v
Open →Impulse
Class 9 Physics
Hit harder or longer, change momentum
Open →Wavelength
Class 9 Physics
Speed over frequency gives wavelength
Open →Oscillations
Class 9 Physics
Vibrate at a frequency for a time
Open →Cost of electricity
Class 10 Physics
Units times rate gives the bill
Open →Number of neutrons
Class 9 Chemistry
Take protons away from the mass number
Open →Curved surface area of a cone
Class 9 Maths
Wrap the slanted side of a cone
Open →Total surface area of a cone
Class 9 Maths
Add the base circle to the cone's side
Open →Curved surface area of a hemisphere
Class 9 Maths
Cover the dome of a hemisphere
Open →Diagonal of a square
Class 9 Maths
Cross a square corner to corner
Open →Unit conversion (m/s to km/h)
Class 9 Physics
Convert m/s into km/h
Open →Distance from velocities
Class 9 Physics
From two speeds, find the distance
Open →Diagonal of a rectangle
Class 9 Maths
Cross a rectangle corner to corner
Open →Diagonal of a cuboid
Class 9 Maths
The longest rod that fits in a box
Open →Area by Heron's formula
Class 9 Maths
Area from just the three sides
Open →Interior angle sum of a polygon
Class 8 Maths
Add up a polygon's inside angles
Open →Exterior angle of a regular polygon
Class 8 Maths
Share 360 among a polygon's corners
Open →Number of diagonals of a polygon
Class 8 Maths
Count the diagonals of a polygon
Open →Discriminant
Class 10 Maths
Test how many roots a quadratic has
Open →Sum of roots
Class 10 Maths
Sum of a quadratic's roots
Open →Punnett square (monohybrid cross)
Class 10 Biology
Cross two parents, predict the offspring
Open →Balancing chemical equations
Class 10 Chemistry
Slide coefficients until atoms balance
Open →Writing chemical formulae (valency)
Class 9 Chemistry
Criss-cross valencies into a formula
Open →Current from power
Class 10 Physics
Divide power by voltage for current
Open →Power (P = V^2 / R)
Class 10 Physics
Voltage squared over resistance
Open →Sine ratio
Class 10 Maths
Opposite over hypotenuse
Open →Cosine ratio
Class 10 Maths
Adjacent over hypotenuse
Open →Tangent ratio
Class 10 Maths
Opposite over adjacent
Open →Area of an equilateral triangle
Class 9 Maths
Area of an equilateral triangle
Open →Curved surface area of a cylinder
Class 9 Maths
Wrap only the curved side
Open →Loss percentage
Class 8 Maths
Sell below cost, find the loss %
Open →Amount with simple interest
Class 8 Maths
Principal plus its simple interest
Open →Distance formula
Class 10 Maths
Straight distance between two points
Open →States of matter
Class 9 Chemistry
Heat particles solid → liquid → gas
Open →Parts of a plant cell
Class 8 Biology
Tap a cell part to see its job
Open →Diagonal of a cube
Class 9 Maths
Longest diagonal through a cube
Open →Total surface area of a hemisphere
Class 9 Maths
Dome plus its flat circle
Open →Sum of first n natural numbers
Class 10 Maths
Add 1 + 2 + ... + n instantly
Open →Range of data
Class 9 Maths
Spread from smallest to largest
Open →Class mark
Class 9 Maths
Midpoint of a class interval
Open →Selling price from profit percent
Class 8 Maths
Mark up cost by a profit %
Open →Perimeter of a sector
Class 10 Maths
Two radii plus the curved arc
Open →Circumference from diameter
Class 8 Maths
Circumference straight from diameter
Open →Power (P = F x v)
Class 9 Physics
Force times velocity gives power
Open →Percentage of a number
Class 8 Maths
Find a percentage of a number
Open →Series and parallel circuits
Class 10 Physics
Break a bulb in series vs parallel
Open →Symbols of elements
Class 9 Chemistry
Match each element to its symbol
Open →Free fall (velocity)
Class 9 Physics
Drop from a height, hit this speed
Open →Free fall (time)
Class 9 Physics
How long a drop takes
Open →Free fall (distance)
Class 9 Physics
Distance fallen in a given time
Open →Complement of an event
Class 10 Maths
Chance an event does NOT happen
Open →Product of roots
Class 10 Maths
Product of a quadratic's roots
Open →Exterior angle theorem
Class 9 Maths
Exterior angle = sum of remote interiors
Open →Complementary angles
Class 10 Maths
What adds to 90 degrees
Open →Supplementary angles
Class 9 Maths
What adds to 180 degrees
Open →Perimeter of a semicircle
Class 10 Maths
Curved half plus the diameter
Open →Area of a semicircle
Class 10 Maths
Half the area of a circle
Open →Turning effect (moments)
Class 9 Physics
Balance the see-saw with moments
Open →Reflex arc
Class 10 Biology
Step through a reflex, stimulus to action
Open →Buoyant force (upthrust)
Class 9 Physics
Displace liquid, feel the upthrust
Open →Relative density
Class 9 Physics
Compare a density to water's
Open →Power in lifting a load
Class 9 Physics
Lift a load, faster needs more power
Open →Cosecant ratio
Class 10 Maths
Hypotenuse over opposite
Open →Secant ratio
Class 10 Maths
Hypotenuse over adjacent
Open →Cotangent ratio
Class 10 Maths
Adjacent over opposite
Open →Height from angle of elevation
Class 10 Maths
Height from an angle of elevation
Open →Area of a quadrant
Class 10 Maths
A quarter of a circle's area
Open →Interior angle of a regular polygon
Class 8 Maths
One inside angle of a regular polygon
Open →Sum of first n odd numbers
Class 10 Maths
Add the first n odd numbers
Open →Sum of first n even numbers
Class 10 Maths
Add the first n even numbers
Open →Quadratic formula (a root)
Class 10 Maths
Larger root of a quadratic
Open →LCM from HCF
Class 10 Maths
LCM from the product and HCF
Open →Depreciation
Class 8 Maths
Value drops by a % each year
Open →Cost price from selling price
Class 8 Maths
Work back to the cost price
Open →Downstream speed
Class 8 Maths
Row with the current
Open →Upstream speed
Class 8 Maths
Row against the current
Open →Average speed for a round trip
Class 8 Maths
Average speed there and back
Open →Sales tax / GST
Class 8 Maths
Tax added on a price
Open →Area of a ring (annulus)
Class 10 Maths
Area of a ring between two circles
Open →Edge of a cube from volume
Class 9 Maths
Edge back from the volume
Open →Radius from area
Class 10 Maths
Radius back from a circle's area
Open →Side from area of a square
Class 8 Maths
Side back from a square's area
Open →Height of a triangle from area
Class 9 Maths
Height back from area and base
Open →Rate from simple interest
Class 8 Maths
Rate back from the interest
Open →Time from simple interest
Class 8 Maths
Time back from the interest
Open →Principal from simple interest
Class 8 Maths
Principal back from the interest
Open →Mean proportional
Class 10 Maths
Geometric mean of two numbers
Open →Fourth proportional
Class 8 Maths
Complete the proportion a : b = c : ?
Open →Marked price from selling price
Class 8 Maths
Marked price back from the sale price
Open →Chambers of the human heart
Class 10 Biology
Tap a heart chamber to see its job
Open →Equation of a line (y = mx + c)
Class 9 Maths
Read y off a straight line
Open →Average term of an AP
Class 10 Maths
Average of first and last term
Open →Number of terms in an AP
Class 10 Maths
How many terms in an AP
Open →Midpoint of two points
Class 10 Maths
x-coordinate of a midpoint
Open →Empirical mode
Class 10 Maths
Estimate the mode from mean & median
Open →Length of a shadow
Class 10 Maths
Shadow from height and sun angle
Open →Train crossing a pole
Class 8 Maths
Speed to cross a pole
Open →Time and work
Class 8 Maths
More workers, fewer days
Open →Dividing in a ratio
Class 8 Maths
Split a total in a ratio
Open →Unitary method
Class 8 Maths
Cost of a single item
Open →Your progress — Foundation - Class 8 to 10
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Showing 14 questions for Class 9. Tap a card to reveal the answer.
ChemistryAtoms and MoleculesmediumState 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.
ChemistryAtoms and MoleculesmediumHow many moles are there in 36 g of water (molar mass 18 g/mol)? Slide to explore.
Reveal answer ↓
What it is
The mole links the mass of a substance to the number of particles it contains.
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.
ChemistryAtoms and MoleculesmediumHow 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.
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.
ChemistryAtoms and MoleculesmediumHow 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.
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).
ChemistryAtoms and MoleculesmediumWhat 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.
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.
ChemistryAtoms and MoleculesmediumHow 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.
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.
ChemistryAtoms and MoleculeseasyCan 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.
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).
ChemistryAtoms and MoleculesmediumState 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.
ChemistryAtoms and MoleculesmediumState 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 and MoleculesmediumDefine 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).
ChemistryAtoms and MoleculesmediumWhat 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.
ChemistryAtoms and MoleculesmediumWrite 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.
ChemistryAtoms and MoleculeshardHow 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.
ChemistryAtoms and MoleculesmediumCalculate 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.
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