C onsider the following situations of daily life and think what happens when –

• milk is left at room temperature during summers.
• an iron tawa/pan/nail is left exposed to humid atmosphere. grapes get fermented.
• food is cooked.
• food gets digested in our body.
• we respire.

In all the above situations, the nature and the identity of the initial substance have somewhat changed. We have already learnt about physical and chemical changes of matter in our previous classes. Whenever a chemical change occurs, we can say that a chemical reaction has taken place.
You may perhaps be wondering as to what is actually meant by a chemical reaction. How do we come to know that a chemical reaction has taken place? Let us perform some activities to find the answer to these questions.