Acid A substance that contains a positively charged hydrogen atom (ion). An acid will always give up its hydrogen ion in order to rid itself of its positive charge and become neutral. Alternatively, it can accept an electron, which is negative, to achieve the same result. This means that acids readily react with other substances.

Actinides A group of radioactive metals. They are transition metals.

Adhesion The sticking together of different particles or surfaces. 

Alkali A base that dissolves in water. An alkali is a substance that has an overall negative charge. It is always willing to give up an electron or receive a proton in order to become neutral. This means that it readily reacts with other substances. All alkalis are bases.

Alkali metals A group of metals that react with water to form alkaline solutions. All alkali metal atoms have just one electron in their outer shell. Alkali metals are generally soft, shiny and a pale silvery colour. Lithium, sodium and potassium are all members of this group.

Alkaline earth metals A group of alkali metals that have two electrons in their outer shell. Magnesium, calcium and radium are all members of this group.

Allotrope Different forms of the same element. For example, diamond and graphite are both allotropes of the element carbon.

Alloy A mixture of two or more elements, of which at least one is a metal.

Aluminium A light, soft metal that does not rust. It easily bonds with other elements to make alloys that are lightweight and durable. It is the commonest metal found in the Earth’s rocky crust.

Atom A basic building block of matter. Atoms are not solid but are mostly made up of empty space. At the centre of the atom, the subatomic particles protons and neutrons are clumped together to form the nucleus. Surrounding this are smaller electrons, which move at speed around the nucleus.

Atomic mass The average mass of one atom of an element compared with the mass of one carbon atom.

Atomic number The number of protons found in the nucleus of an atom.

Base A substance that reacts with an acid to form a salt and water.

Bond A link between atoms.

Bronze A strong, shiny, brown alloy of tin and copper. Bronze was the first alloy to be used by humans. It was used to make weapons, tools and ornaments.

Carbon One of the most common elements in the Universe and the main element in all living things.

Carbon dioxide A colourless gas that is produced by animals when they breathe out. It also emitted naturally from volcanoes and during the process of decay of once-living things. It is also the product of burning of wood and fossil fuels. Plants take in carbon dioxide as part of photosynthesis. It is also dissolved in ocean waters. Its chemical formula is CO₂.

Catalyst A substance that triggers or speeds up chemical reactions.

Chemical reactions A chemical reaction takes place when one substance joins with another to form one or more completely new substances. Chemical reactions are taking place all around us. Some happen naturally. For example, the food we eat is chemically changed inside our bodies to produce energy. Other reactions are made to take place artificially, for example in the manufacture of medicinal drugs.

Chemical symbols A letter or letters used to represent a chemical element. For example, C represents carbon and Cu stands for copper.

Chemistry Chemistry is the study of the composition, structure and properties of matter and the reactions between substances.

Cohesion The sticking together of surfaces or particles of the same kind. 

Combustion A chemical reaction in which oxygen combines with a fuel, releasing energy. This is otherwise known as burning. Combustion is a type of oxidation.

Compound A substance formed by more than one element. For example, water is a compound made up of hydrogen and oxygen atoms.

Concentration A measure of how strong a solution is.

Condensing The process of a gas changing into a liquid. Condensation occurs when a gas cools down or is subject to an increase in pressure.

Copper A shiny, pinkish-brown metal. Copper is a good conductor of electricity and is often used to make wires and pipes.

Corrosion The change of a useful metal into another substance due to combination with another element, such as when steel reacts with oxygen from the air to form iron oxide.

Covalent bond A bond between atoms, where one atom shares an electron with another atom.

Crystal A solid substance with a geometric shape. A crystal’s atoms are arranged into a pattern. Crystals have sharp edges and flat sides at regular angles to each other.

Density  A measure of how much a particular volume of a substance weighs. The more
massive the atoms or molecules of a substance are, and the more closely packed they are, the denser the substance is.

Diffusion The process by which molecules move around to fill a space. For example, molecules of ink will diffuse, or spread out, in a glass of water so that the liquid changes colour.

Electrolysis The process of separating the elements in a compound by using an electrical current. The compound must be in a liquid state for this to happen.

Electron A subatomic particle that circles the nucleus of an atom. Electrons do not move at random, but stay in layers known as shells. Electrons have a negative electrical charge, while protons carry a positive charge. Positive and negative charges attract each other, so the atom is held together.

Element An element is a substance made up of atoms of the same type. It cannot be broken down into simpler substances. Different elements have different characteristics, which are determined by the number of subatomic particles they have. There are 94 natural elements known to us. These are divided into metals and non-metals. If atoms of one element join with atoms of another element, they form a new substance. This process is called a chemical reaction. The substance created is called a compound.

Evaporation The process of a liquid changing into a gas. Evaporation occurs when a liquid heats up or experiences a drop in pressure.

Filtration A way of removing solids from liquids by pouring a mixture through a material known as a filter. This is a material with very fine holes in it. It allows liquids, but not solids, to pass through it.

Fluid A substance that has no fixed shape but changes shape to fill its container. Liquids and gases are both fluids.

Freezing The process of a liquid changing into a solid. Freezing occurs when a liquid cools down or is subject to an increase in pressure.

Gas A state of matter in which molecules are separate from each other and can move about freely. This is why gases have no fixed shape and can expand or contract depending on the size of their container.

Gold A soft, shiny, yellow element. Gold is often found in small lumps, called nuggets. It is valuable because it is rare and does not easily spoil or rust. It is a good electrical conductor.

Halogens A group of non-metallic elements with similar characteristics. They easily form bonds with other elements. The halogens are: chlorine, fluorine, bromine, iodine and astatine.

Helium One of the noble gases. Helium is one of the lightest of all gases. It is used to fill party balloons.

Hydrogen The commonest element in the Universe. It is the simplest and lightest chemical element because each atom contains one proton and one electron but no neutrons. Hydrogen joins with oxygen to make water. It also joins with carbon and oxygen to make carbohydrates, found in starchy foods such as potatoes and rice.

Indicator A substance that shows how acidic or alkaline a solution is. Universal indicator and litmus paper are both examples of indicators. They change colour depending on the acidity of a solution. Acidity is measured using the pH scale. A very acidic solution has a pH of 0. A very alkaline solution has a pH of 14. A neutral solution (such as water) has a pH of 7.

Ion Atoms that are positively or negatively charged are known as ions. An atom that loses electrons becomes positive while an atom that gains electrons becomes negative. Positive ions, known as cations, and negative ions, anions, attract and form strong ionic bonds.

Ionic bond A bond between atoms, where an electron leaves one atom and joins another. An atom that loses an electron becomes positively charged, while an atom that gains an electron becomes negatively charged. Atoms that have become positively or negatively charged are known as ions. Positive ions attract negative ions. This keeps the atoms bound together.

Iron A heavy, silvery metal. Iron is a popular construction metal because it is abundant and strong. It is used to make steel and can be used as a magnet. Iron is a vital element needed by the human body: iron inside our red blood cells helps to carry oxygen around our bodies.

Irreversible change A change, or reaction, that cannot be undone. When you bake a cake, the molecules in its ingredients are rearranged. You cannot change the cake back into its ingredients. Burning is another type of irreversible change. Irreversible changes always produce a new substance.

Isotopes The atoms of an element that has the normal number of protons and electrons, but different numbers of neutrons. Isotopes have the same atomic number, but different atomic masses. The different isotopes of an element have identical chemical properties, but some isotopes are radioactive. Deuterium is an isotope of hydrogen. It contains a proton and a neutron in its nucleus, while normal hydrogen has only a proton. Carbon-12, carbon-13 and carbon-14 are three isotopes of carbon. The first two are stable isotopes, but carbon-14 is radioactive.

Lanthanides A group of silvery metals with very high melting points. They are sometimes called the “rare earth metals”. They belong to the transition metals.

Lead A heavy, bluish metal that turns grey when exposed to the air and has a very low melting point. Lead is often made into weights because it is so heavy.

Liquid A state of matter in which molecules are close to each other but not fixed rigidly together. The molecules can move about to fill the bottom of their container. Liquids can be easily poured.

Magnesium A light, silvery-white metal. Magnesium easily reacts with other elements and burns with a bright white flame. It is used to make fireworks or mixed with other metals to make alloys that are strong but light.

Mass The amount of matter in an object.

Matter Every thing around us, living creatures, lumps of rock, or even invisible gases, is made up of matter. Matter exists in three main forms, or states. These are: solid, liquid and gas. All matter is made up of tiny building blocks called atoms. They are far too small for us to see and are themselves made up of even tinier parts called subatomic particles.

Melting The process of a solid changing into a liquid. Melting occurs when a solid heats up and reaches its “melting point”.

Mercury A silvery liquid metal. Mercury is the only metal that is a liquid at room temperature. It is sometimes used inside thermometers.

Metallic bond A bond between atoms in a metal. Some of he metal’s electrons flow freely from one atom to another.

Metals Metals are the largest group of chemical elements, making up about three-quarters of the Periodic Table. Metals conduct heat and electricity well compared to non-metals. Most are solid at room temperature. They are strong, hard and tough, and can be polished to give a smooth, shiny surface. When they are squeezed under great pressure, they change shape and become squashed, rather than splinter apart or shatter. These features are true of most metals, but not all. For example, the metal sodium is very soft, while mercury is a silvery liquid at room temperature.

Miscibility The ability of liquids to mix together. Miscible liquids mix easily. Immiscible liquids, for example oil and water, will not mix. If an emulsifier is added, immiscible liquids will float together in a mixture called an emulsion.

Mixture A blend of two different substances that are not chemically changed by being put together. Sand and salt combined produce a mixture—neither the salt nor the sand are chemically changed.

Molecule The smallest part of a substance that can exist by itself and still possess its chemical properties. A molecule consists of atoms, either of the same type, or a combination of different types of atom bonded together with covalent bonds.

Neutralize To turn an acid or alkali into a neutral solution—one that is neither acid nor alkaline. This can be achieved by mixing an acid and an alkali together. The acid donates its positive hydrogen atom to the negative alkali, so that they both have a neutral charge.

Neutron A subatomic particle found in the nucleus of an atom. Neutrons have no electrical charge.

Nickel A silvery-white metal. It is used to make stainless steel and other alloys. It is also used to make some coins.

Nitrogen A colourless, odourless gas that makes up 78% of the air around us. Nitrogen is essential for life because it makes up a part of all living cells.

Noble gases A group of gases that have similar characteristics. They are sometimes known as the inert gases because they do not easily form compounds with other elements. One common noble gas is neon, which is used to make lights and street signs. Because it does not react with other elements, it will not ignite or explode.

Nucleus The centre of an atom, made up of protons and neutrons. The centre of a living cell is also called a nucleus.

Ore Rock from which metals or other useful substances can be extracted.

Oxidation A reaction in which oxygen combines with another chemical.

Oxygen A colourless, odourless gas that makes up one-fifth of the air around us. Oxygen is crucial to much life on Earth as humans and other animals must breathe oxygen to stay alive.

Periodic Table The Periodic Table of elements is a list of all known elements arranged in order of their atomic numbers (the number of protons in each atom). Elements with similar chemical properties are placed in the same group. The Periodic Table was designed by Russian scientist Dmitri Mendeleev (1834–1907).

Phosphorus A chemical element that is found in many rocks and minerals. It is also found inside our bones and DNA. In its pure form, phosphorus is highly flammable.

Physical change A change in a substance that affects its appearance but not its chemical properties. Dissolving sugar in hot water and melting an ice cube are both examples of physical changes. The sugar molecules spread through the water and the ice changes state, but they are otherwise unaltered.

Plasma A state of matter rare on Earth, found at very high temperatures. For example, plasma is found inside most stars, including our local star, the Sun. A gas becomes a plasma when it is so hot that its atoms collide with such force that electrons are knocked loose from some atoms, so that the gas becomes electrically charged.

Plastic A man-made, or synthetic material, that is made in laboratories or factories. Plastics are useful materials because they are easy to shape and are often light and long-lasting. Most plastics are made from compounds of hydrogen and carbon and are a type of polymer.

Plutonium A highly radioactive metal. Plutonium is used to produce nuclear energy.

Polymer A large molecule made from chains of smaller molecules called monomers. All types of plastic are polymers. Natural polymers are found in our bodies, for example, DNA.

Poor metals The name sometimes given to a group of soft, weak metals that melt at low temperatures. Tin, lead and aluminium are all poor metals.

Precious metals Valuable metals such as gold, silver and platinum. They are prized for their rarity and for their beautiful appearance. They are often used to make jewellery and other ornaments.

Properties of matter The characteristics of a substance. Chemical properties are characteristics that can only be discovered through chemical reactions. Physical properties are characteristics that can be measured, such as hardness, melting point and density.

Proton A subatomic particle found in the nucleus of an atom. Protons have a positive electrical charge.

Radioactivity Particles or electromagnetic waves given off by unstable atoms, for example, uranium. An unstable atom is one that is likely to give off one of its subatomic particles.

Reactant Reactants are the starting materials in a chemical reaction that are consumed during the reaction. Reactants undergo a chemical change in which their chemical bonds are broken and new ones formed to make products. Although they are involved in the reaction, solvents are not usually called reactants.

Reactive How likely a substance is to join with another substance in a chemical reaction.

Reversible change A change, or reaction, that can be undone. If you freeze water, you can reverse the change by melting the ice. If you dissolve salt in water, you can boil off the water to leave the salt. Reversible changes never create new substances.

Salt A solid compound made when a base joins with an acid. A salt is a type of crystal. Many types of salt dissolve in water. Table salt is called sodium chloride, but saltpetre, or nitre (potassium nitrate), is also a type of salt.

Semi-metals Otherwise known as the metalloids, the semi-metals are a group of elements that are neither metals nor non-metals. Typically they have the hard, shiny appearance of a metal, but make poor electrical conductors. Arsenic and bismuth are both semi-metals.

Silver A soft, bright, shiny metal. Silver is a precious metal, used to make ornaments and jewellery. It is also a good conductor of electricity.

Solid A state of matter in which molecules are tightly packed together in a rigid pattern. This gives solids a fixed shape.

Soluble The ability of one substance to dissolve in another one.

Solution A liquid mixture of two different substances. One substance, known as the solute, usually a solid, is dissolved in another substance, known as the solvent, which is usually a liquid. For example, table salt stirred into water is a solution.

States of matter The four forms in which matter exists. These are: solid, liquid, gas and plasma. A change of state is caused by a change in temperature or pressure.

Steel An alloy of iron and carbon. There are many different types of steel alloys, each containing slightly different amounts of various elements, and each designed to do a different task. Stainless steel is a mixture of steel, chromium, and small amounts of other metals. It is used to make sinks and cutlery. Titanium and vanadium steels resist high temperatures without melting.

Subatomic particles The tiny parts that make up an atom. They include electrons, neutrons and protons.

Sublimation The process of a solid turning straight into a gas without first becoming a liquid.

Sulphur An element that is found in some rocks. It readily forms bonds with other elements. It is used in gunpowder and matches because it easily catches alight.

Tin A silvery metal that can easily be bent and shaped. Tin was once used to make cans and foil, but aluminium has since taken its place, because it is cheaper.

Transition metals A group of metals that are mostly hard, dense and shiny and make good electrical conductors. Gold, silver, iron and copper are all transition metals.

Vacuum A place where there is a complete absence of matter.

Valency The number of bonds an atom can possibly make with other atoms.

Viscosity A measure of how thick a liquid is. For example, honey is thicker and more viscous than water.

Volume The amount of space taken up by an object.

Zinc A bluish-white metal. It is used to make battery casings. Zinc is also an important part of our diet and is found in foods such as nuts and red meat.

Consultant: Mike Goldsmith