The billions of substances in the universe are all made up of just 100 or so elements. By understanding how the elements combine to form new substances, scientists can develop useful materials such as plastics, perfumes and medicines. Scientist can also synthesis natural materials such as vitamins, flavors and even blood in the laboratory.

Compounds

Most substances you encounter every day are made up of more than one type of atom. These forms of matter are known as compounds. Like elements, every compound has a unique set of characteristics properties. The properties of compounds are usually very different from the properties of the elements that make them up.

Some compounds are made up of molecules- such as water, wax and vegetable oil. In these compounds, each molecule contains two or more types of atoms.

Molecular compounds

All the molecules in a compound are identical in size, shape and number of atoms. Many molecules are small and contain just a few atoms. For example a molecular of carbon monoxide (formula CO) has just one carbon (C) atom and one oxygen (O) atom. The molecules in other compounds, such as some plastics, contain thousands of atoms.

Despite all having very different properties, each molecule is made by combining different numbers of the same elements: oxygen, hydrogen and carbon.

Compound lattices

Compounds can also form crystal lattices. In these lattices, the atoms are bonded very strongly to each other, so they tend to be very hard solids at room temperature. Two common examples of compound lattices are table salt and beach sand.

Beach sand has the scientific name silica or silicon dioxide. Its chemical formula is SiO2. This formula indicates that in a grain of beach sand there is one silicon atom for every two oxygen atoms.

You can see the structure of silicon dioxide,. Silicon dioxide also makes up minerals like quartz, amethyst and opal. It is the main ingredient in glass, and so it is used in optical fibers to carry information such as cable TV and broadband internet.

Mixtures

Many of the substances used every day are not simply elements or compounds but are mixtures of elements and compounds. For example, air is a mixture of the elements oxygen (O2) and nitrogen (N2) with compounds such as water vapor (H2O), carbon dioxide (CO2) and carbon monoxide (CO). Likewise, water from the top is not pure but a mixture.

Unlike elements and compounds, you cannot write chemical formulas for mixtures. This is because mixtures are made up of molecules and crystal lattices.

Everyday mixtures

There are many different types of mixtures that you use everyday. One common type of mixture is a solution. A solution is formwed when a solid dissolves in a liquid. In a solution**:**

• the dissolving solid is known as the solute
• the liquid in which the solute dissolves is known as the solvent.

Many common solutions have water as their solvent. These water- based mixtures are referred to as aqueous solutions. For example, sugar dissolves in water to form an aqueous sugar solution- sugar is the solute and water is the solvent.

If the solid is inviolable (doesn’t dissolve) then the mixture formed is either a colloid or a suspension. In a colloid, the solid particles are so small that they never settle to the bottom of the liquid. Blood and milk are examples of colloids. In a suspension, the solid particles are larger and heavier, and so they will eventually settle at the bottom. A mixture of sand and water is an example of a suspension.

Some liquids can also dissolve to form solutions. Liquids that dissolve in another liquid are said to be miscible. For example, detergent is miscible in water and will dissolve to form a detergent solution.

Alloys

Alloys are mixture of a metal with other metals or non-metals. Alloys are particular important because they often have very different and useful properties compared to the pure metal. For example, iron is cheap to produce but is soft and rusts easily, making it less useful. Adding small amounts of carbon to iron produces the alloy steel, which is strong and useful for building skyscrapers, bridges, machinery and cars,. Adding chromium and nickel produces stainless steel, which stops it rusting.