Chemistry

Organization and properties of elements, patterns in the periodic table, and element families.

The alkali metals are the highly reactive metals in Group 1 of the periodic table. They react vigorously with water and air, producing hydrogen gas and metal hydroxides. Their reactivity increases going down the group.

Every element has a one- or two-letter symbol used worldwide. Chemical formulas combine these symbols with numbers to show exactly which atoms are present in a substance and how many of each.

The periodic table is arranged in vertical columns called groups and horizontal rows called periods. An element's position tells you about its structure and properties.

The halogens are reactive non-metals in Group 7. They exist as diatomic molecules and become less reactive going down the group. Halogens react with metals to form salts, and are used in water purification, disinfectants, and plastics.

Metals are elements on the left and centre of the periodic table. They share characteristic properties such as shininess, electrical conductivity, malleability, and high melting points, which make them useful in everyday life.

Noble gases are the elements in Group 18 of the periodic table. They are colourless, odourless gases that are extremely unreactive because their outer electron shells are completely full.

Non-metals are found on the right side of the periodic table. They are generally poor conductors of heat and electricity, often exist as gases at room temperature, and their solid forms are brittle — yet they are essential to life and many chemical reactions.

The periodic table is a chart of all known elements arranged in order of atomic number. Elements in the same column share similar chemical properties, which makes the table an extremely useful tool in chemistry.

The transition metals are found in the central block of the periodic table. They are strong, dense, and good conductors with high melting points. Unlike Group 1 and Group 7 elements, they are much less reactive and form coloured compounds with multiple oxidation states.

An acid donates hydrogen ions in solution, has a pH below 7, and reacts with metals and bases.

A chemical element is a pure substance made of only one type of atom, identified by its atomic number.

Key definitions and terms across chemistry topics for quick lookup and concept reinforcement.

An atom is the smallest unit of an element, made up of protons, neutrons, and electrons.

A mixture combines two or more substances physically without chemical bonding, separable by physical means.

A chemical reaction transforms reactants into new products through the breaking and forming of chemical bonds.

Study of atomic structure, states of matter, and particle behavior forming the foundation of all chemistry.

Atoms are the tiny particles that make up all matter. Every element is composed of one type of atom, and atoms are too small to see with the naked eye or even an ordinary microscope.

A chemical experiment is a planned investigation that tests a hypothesis using controlled conditions. Learning to design, carry out, and record experiments is a fundamental scientific skill.

Chemistry is not confined to laboratories — it is happening all around you, every moment of the day. From the food on your plate to the screen you are reading this on, chemistry explains how the world works.

The chemistry laboratory is where scientists design and carry out experiments. Understanding its equipment and safety rules is essential before any practical work can begin.

Electrons occupy shells (energy levels) around the nucleus. The arrangement of electrons in these shells — called electronic configuration — determines how an element behaves chemically and reacts with other elements.

An element contains only one type of atom and cannot be broken down into simpler substances. A compound contains two or more different elements chemically bonded together and has different properties from the elements it was made from.

An ion is a charged atom or group of atoms that has gained or lost electrons. When metals and non-metals react, they transfer electrons to form positive and negative ions that attract each other — this is ionic bonding.

Isotopes are atoms of the same element that have different numbers of neutrons. They have the same chemical properties but different masses. Some isotopes are radioactive, while others are stable.

Molecules are formed when two or more atoms bond together. They can be made of atoms of the same element or different elements, and their properties differ from the individual atoms they are made of.

Every atom has a nucleus containing protons and neutrons, surrounded by electrons. The number of protons determines which element an atom belongs to and gives it its chemical identity.

Chemistry is the science of matter — what substances are made of, how they behave, and how they change. It underpins almost everything in the world around us, from the food we eat to the medicines that keep us healthy.

Air is not a single substance — it is a mixture of several gases. Understanding what air is made of and what each component does helps us understand breathing, weather, and life on Earth.

Human activity has changed the composition of Earth's atmosphere in ways that harm both the environment and human health. Understanding air pollutants and their effects is the first step toward solving these problems.

Matter changes from one state to another when energy (heat) is added or removed, causing particles to move faster or slower — these changes have specific names like melting, freezing, evaporation, and condensation.

Density is a measure of how much mass is packed into a given volume. Dense materials have particles packed closely together. Density explains why some objects sink and others float, and why gases are much lighter than solids or liquids.

Diffusion is the net movement of particles from an area of high concentration to an area of low concentration. It happens because particles are always moving randomly, and explains how smells travel, how oxygen moves into blood, and how substances move in and out of cells.

Gas pressure is caused by gas particles colliding with the walls of their container. Pressure increases when particles move faster (higher temperature) or are squeezed into a smaller space (lower volume). Understanding gas pressure is essential in everyday applications from tyres to weather systems.

Gases have no fixed shape or volume — their particles move rapidly in all directions and spread out to fill whatever container they are in.

Liquids have a fixed volume but no fixed shape — their particles are close together but free to move past each other, allowing liquids to flow and take the shape of their container.

Every substance has a specific melting point and boiling point — temperatures at which it changes state. These fixed temperatures are used to identify pure substances.

Solids have a fixed shape and volume because their particles are tightly packed in a regular arrangement and can only vibrate in place.

Water is one of the most remarkable substances on Earth. Its unique physical and chemical properties make life possible and shape our entire planet.

Study of mixtures, solutions, and separation techniques for understanding combinations of substances.

Chromatography separates mixtures of dissolved substances based on how strongly they are attracted to a stationary material compared to how far they are carried by a moving solvent. It is used to identify unknown substances and test for purity.

Concentration tells us how much solute is dissolved in a given volume of solvent. Understanding concentration is essential in laboratory experiments, medicine, cooking, and industry — it affects the rate of reactions and the strength of solutions.

Crystallisation is a method of separating a dissolved solid from a solution by evaporating the solvent — usually water — until crystals grow. It is used to purify substances and produce high-quality crystals.

Distillation separates a liquid from a solution or separates two mixed liquids by heating the mixture so one liquid evaporates, then cooling the vapour so it condenses and is collected separately.

Evaporation is used to separate a dissolved solid (solute) from its solution by heating the solution so the solvent evaporates away. The solid solute is left behind as pure crystals.

Filtration is a separation technique used to separate an insoluble solid from a liquid (or gas) by passing the mixture through a filter. The liquid passes through while the solid is retained on the filter.

Master the three types of mass fraction calculation — finding the fraction, finding the solute mass, and finding the solution mass — with fully worked step-by-step examples.

Mass fraction tells you what proportion of a mixture is made up of one particular substance. Learn the formula, how it differs from concentration, and where you encounter mass fractions every day.

A mixture contains two or more substances that are not chemically combined. The substances keep their own properties and can be separated by physical methods such as filtering, evaporation, or distillation.

A pure substance contains only one type of substance with no other materials mixed in. Pure substances have sharp, fixed melting and boiling points, while impure substances (mixtures) melt and boil over a range of temperatures.

Solubility is the maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature. Temperature usually increases the solubility of solids, while it decreases the solubility of gases.

A solution is a homogeneous mixture formed when a solute dissolves in a solvent. Water is the most common solvent. The resulting solution looks clear and uniform throughout.

Not all mixtures are true solutions. Suspensions contain large insoluble particles that settle over time, while colloids have smaller particles that remain evenly dispersed. These mixtures are common in food, medicine, and everyday products.

Fresh water must be treated before it is safe to drink. This article explains how water is purified from natural sources to your tap, and why clean water is one of humanity's most important resources.

Acid rain forms when sulfur dioxide and nitrogen oxides from burning fossil fuels dissolve in atmospheric water to make sulfuric and nitric acid. It damages ecosystems, corrodes buildings, and harms aquatic life.

Acids are substances that produce hydrogen ions (H⁺) in water. They taste sour, react with metals to produce hydrogen gas, react with carbonates to produce carbon dioxide, and have pH values below 7.

Bases are substances that neutralise acids. An alkali is a base that dissolves in water, producing hydroxide ions (OH⁻). Alkalis have a pH above 7 and feel slippery to touch.

Acids and alkalis are all around us — in food, cleaning products, our bodies, and the environment. Knowing the pH of a substance and understanding the hazards of strong acids and alkalis helps us use them safely.

Indicators are substances that change colour depending on whether a solution is acidic or alkaline. They are used in chemistry to identify acidic or alkaline conditions and to find the endpoint of a neutralisation reaction.

Neutralisation is the reaction between an acid and a base (or alkali) to produce a salt and water. The pH of the mixture moves towards 7 (neutral) as the reaction occurs.

The pH scale measures how acidic or alkaline a solution is, running from 0 (most acidic) to 14 (most alkaline), with 7 being neutral. It is based on the concentration of hydrogen ions in the solution.

A chemical reaction transforms reactants into entirely new substances called products. Learn to recognise reactions by their signs and understand why matter is always conserved.

Chemical synthesis combines simpler substances to build more complex ones. Discover how this fundamental type of reaction underpins everything from fertilisers to life-saving medicines.

A word equation summarises a chemical reaction by naming the reactants on the left and the products on the right, linked by an arrow. It is the simplest way to describe what happens in a reaction.

Combustion is a chemical reaction in which a fuel reacts rapidly with oxygen, releasing heat and light. Complete combustion of a hydrocarbon fuel produces carbon dioxide and water.

In any chemical reaction, the total mass of the reactants equals the total mass of the products. Atoms are rearranged but never created or destroyed — this is the law of conservation of mass.

A displacement reaction is when a more reactive element takes the place of a less reactive element in a compound. These reactions reveal the reactivity series — the ranking of metals from most to least reactive.

Chemical reactions either release heat energy into the surroundings (exothermic) or absorb heat energy from the surroundings (endothermic).

Chemical reactions always involve energy changes. Exothermic reactions release energy (feel hot), while endothermic reactions absorb energy (feel cold). Understanding these changes is essential for industry, food, and everyday life.

Four key factors control how fast a chemical reaction proceeds — temperature, concentration, surface area, and catalysts. Learn how each one works and why it matters in everyday life.

Every fire-extinguishing method works by removing one side of the fire triangle. Learn how different extinguishers work, which fires they suit, and how to stay safe at home and school.

Fire is not a substance but a chemical reaction called combustion. Learn what fire really is, how the fire triangle works, and what different flame colours tell us.

Careful observation is a fundamental skill in chemistry. Learn what scientists look for when recording a chemical reaction and practise systematic experimental observation.

Oxidation is the reaction of a substance with oxygen. Rusting is the oxidation of iron — a slow reaction that forms iron oxide when iron, water, and oxygen are all present together.

A physical change alters the appearance of a substance but not its chemical identity, while a chemical change produces a new substance with different properties. Knowing the difference is fundamental to understanding chemistry.

A precipitation reaction produces an insoluble solid (precipitate) when two solutions are mixed. The precipitate forms because one of the possible products cannot dissolve. These reactions are used to identify ions and remove pollutants from water.

Reaction rate tells us how fast a chemical reaction produces new substances. Find out how chemists measure speed, what activation energy is, and why some reactions take seconds while others take years.

Thermal decomposition is a reaction in which a single compound breaks down into two or more simpler substances when heated. It is a common type of chemical change and is used in industry to extract metals and make lime.

Study of chemical reactions, energy changes, reaction types, and factors affecting reaction rates.