Master the basics - and everything after gets easier

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.

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.

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).

What it is

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

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.

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.

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.

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.

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.

What it is

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

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.

What it is

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

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.

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.

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.

What it is

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

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.

What it is

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

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.

What it is

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

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.

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.

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.

What it is

To convert a Kelvin temperature to Celsius, subtract 273.

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.

What it is

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

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.

What it is

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

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.

What it is

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

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.

What it is

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

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.

What it is

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

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.

What it is

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.