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.
The chemical substances that you start with in a reaction — the ingredients that get used up.
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:
A change in which no new substance is formed. The material may look different, but its chemical identity stays the same. Many physical changes are reversible, though not all.
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? | ✅ Often yes | ❌ No (or very difficult) |
| Example | Ice melting to water | Wood burning to ash |
| Example | Sugar dissolving in water | Iron rusting |
Dissolving sugar in water is a physical change — the sugar is still sugar, just spread through the water, and you could get it back by evaporating the water. But burning wood is a chemical change — it turns into ash, carbon dioxide, and water vapour, and you absolutely cannot get your wood 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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