What Is a Chemical Reaction?
Every time a new substance is formed from existing ones, a chemical reaction has taken place. During a chemical reaction, bonds between atoms are broken and new bonds form , the starting materials change into something completely different.
Reactants: The chemical substances that you start with in a reaction , the ingredients that get used up.
Products: The new chemical substances that are produced at the end of a reaction , what you finish with.
Think of baking a cake. The flour, eggs, sugar, and butter are your reactants , you put them in. The baked cake is your product , something completely new that you can't easily turn back into its ingredients!
Physical Changes vs Chemical Changes
Not every change that happens to a substance is a chemical reaction. Scientists draw an important distinction between two types of change:
Physical Change: A change in which no new substance is formed. The material may look different, but its chemical identity stays the same. Physical changes are usually reversible.
Chemical Change: A change in which new substances are formed because bonds are broken and new ones are made. Chemical changes are generally difficult or impossible to reverse.
Here's a quick comparison:
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substance formed? | ❌ No | ✅ Yes |
| Usually reversible? | ✅ Yes | ❌ No (or very difficult) |
| Example | Ice melting to water | Wood burning to ash |
| Example | Tearing paper | Iron rusting |
Screwing up a piece of paper is a physical change , the paper is still paper, just a different shape, and you could flatten it out again. But burning that paper is a chemical change , it turns into ash, carbon dioxide, and water vapour, and you absolutely cannot get your paper back!
Throughout this subtopic, we are focusing specifically on chemical changes , how we recognise them and how we represent them using equations.
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