Pure substances and mixtures
Different types of chemical substance
Substances exist as elements, compounds or mixtures:
- An A substance made of one type of atom only. contains The smallest part of an element that can exist. with the same The number of protons in the nucleus of an atom. Also called the proton number..
- A A substance formed by the chemical union of two or more elements. contains two or more elements chemically joined together.
- A Two or more substances that are not joined together. The substances can be elements, compounds, or both. contains two or more different substances that are not chemically joined together. The different substances in a mixture can be elements and/or compounds.
The table shows some examples:
The meaning of pure
The word A substance that consists of only one element or only one compound. is used in chemistry in a different way from its everyday meaning. For example, shops sell cartons labelled as âpureâ orange juice. The label means that the contents are just orange juice, with no other substances added. However, the juice is not pure in the chemical sense, because it contains different substances mixed together. In chemistry:
- a pure substance consists only of one element or one compound
- a mixture consists of two or more different substances, not chemically joined together
Distinguishing between pure substances and mixtures
Pure substances have a sharp The temperature at which a solid changes into a liquid as it is heated. but mixtures The process that occurs when a solid turns into a liquid when it is heated. over a range of temperatures. This difference is most easily seen when the temperature of a liquid is measured as it cools and A change of state in which liquid becomes solid by cooling.. The graph shows the cooling curve for a sample of a compound called salol.
The horizontal part of the graph shows that the salol has a sharp melting point, so it is pure. A substance, usually unwanted, that is present in another substance. salol (a mixture of salol and other substances) would produce a gradual fall in temperature as it freezes.