Making new substances — Chemistry, 7–10 years
Sometimes materials change into substances with new properties. Bubbles, a new colour or a solid appearing can be clues, but we must compare carefully before deciding.
A change that makes something new
In some changes, the starting materials become different substances. The new substances may have different colours, smells, textures or reactions, and the change may be hard to reverse. Burning wood, rust forming on iron and vinegar reacting with bicarbonate are examples of this kind of change.
Why look for new substances?
People needed to tell the difference between a material merely changing shape and one becoming something else. This helps them choose safe materials, understand cooking and burning, and predict whether a change can be undone. Scientists use observations because a new substance cannot be seen directly as it forms.
Vinegar and bicarbonate
1. Put two spoonfuls of bicarbonate of soda in a cup. 2. Add three spoonfuls of vinegar. 3. Bubbles appear and the cup may feel cooler: a gas is forming. 4. After the fizzing stops, the powder and vinegar are no longer simply separate as before. The clues show that a reaction made new substances, including carbon dioxide gas.
Bubbles are not the whole test
It is tempting to say that every bubble proves a new substance has formed. That is reasonable because the gas is easy to notice, but boiling water also makes bubbles without making a new substance. Look at the whole situation: compare before and after, and ask whether heating or another reaction explains what you see.
New substances in ordinary life
Baking uses reactions: heat changes a mixture of ingredients so that the cake sets, rises and smells different. Iron objects can slowly react with oxygen and water to form rust. These changes are useful or unwanted depending on the situation, and they cannot be treated like simply cutting, melting or mixing a material.
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