Chemical reactions — Science, 11–13 years
In a chemical reaction, substances change into new substances with different properties. Looking for evidence of a new material helps separate a reaction from simply mixing or changing shape.
The idea
A chemical reaction rearranges tiny particles into different combinations. The starting substances are called reactants, and the new substances are products. The products may have a new colour, smell, texture or ability, because they are not just the original materials in a new shape.
Why the idea matters
People needed to tell whether a material had merely been moved or whether it had become something new. That matters for cooking, rust, medicines and safe handling of substances. The reaction idea gives us a way to explain lasting changes instead of memorising a list of surprising effects.
Vinegar and bicarbonate
Put 20 ml of vinegar in a cup and add one teaspoon of bicarbonate of soda. Bubbles appear immediately: a gas is forming, so this is evidence of a reaction. The fizzing slows when a starting substance is used up; the mixture is not simply vinegar and powder in a different arrangement.
The common mistake
A reasonable mistake is calling every visible change a chemical reaction. Ice melting and paper being torn look different, but the material has not become a new substance. A stronger clue is a lasting change such as gas forming, a solid appearing or heat being released, though one clue alone is not always proof.
Where it appears
Baking uses reactions to make dough rise and set; batteries use reactions to produce electrical energy; rust forms when iron reacts with oxygen and water. These changes are useful or harmful depending on control, so engineers choose materials and conditions carefully.
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