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CHEMICAL REACTIONS AND EQUATIONS

Consider 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.

Burning of a magnesium ribbon in air and collection of magnesium oxide in a watch-glass
Figure 1.1 Burning of a magnesium ribbon in air and collection of magnesium oxide in a watch-glass

You must have observed that magnesium ribbon burns with a dazzling white flame and changes into a white powder. This powder is magnesium oxide. It is formed due to the reaction between magnesium and oxygen present in the air.

From the above three activities, we can say that any of the following observations helps us to determine whether chemical reaction has taken place –

  • change in state
  • change in colour
  • evolution of a gas
  • change in temperature.

As we observe the changes around us, we can see that there is a large variety of chemical reactions taking place around us.

Formation of hydrogen gas by the action of dilute sulphuric acid on zinc
Figure 1.2 Formation of hydrogen gas by the action of dilute sulphuric acid on zinc

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