Pure Substances (AQA GCSE Chemistry): Revision Notes

When we talk about something being "pure" in everyday life, we usually mean it's free from dirt or any unwanted substances. For instance, if you say you have a glass of pure water, you probably mean it's clean, without any dirt, bacteria, or harmful chemicals. But even this "pure" water will still have other things in it, like dissolved ions, along with the water molecules (H₂O). However, in chemistry, the word "pure" has a much stricter meaning.

For chemists, a pure substance is one that contains only one type of compound or element. So, for water to be chemically pure, it must consist only of H₂O molecules, with absolutely nothing else mixed in.

Chemists, especially those working in fields like medicine, often need to check if a substance is pure. There are two main ways to do this: melting point analysis and boiling point analysis.

1. Melting Point Analysis: This method is used to check the purity of a solid. Here's how it works:

• Crystals of the solid are placed into a tiny tube called a capillary tube.
• This tube is then put into a machine that slowly heats it up.
• As the temperature rises, the chemist watches to see when the solid melts, recording the temperature at that moment. If the substance is pure, it will melt exactly at its melting point. If it's not pure, it will melt at a lower temperature and over a range of temperatures instead of a single, precise point.

2. Boiling Point Analysis:

This method is similar but is used for liquids:

• The liquid is heated, and the temperature is measured as it starts to boil.
• A pure liquid will boil exactly at its boiling point, while an impure one will boil at a higher temperature and, again, over a range of temperatures.