Chemical Reactions and Equations

 

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.


Activity 1.1

CAUTION: This Activity needs the teacher’s assistance. It would be better if students wear suitable eyeglasses.

  • Clean a magnesium ribbon about 3-4 cm long by rubbing it with sandpaper.

  • Hold it with a pair of tongs. Burn it using a spirit lamp or burner and collect the ash so formed in a watch-glass as shown in Fig. 1.1. Burn the magnesium ribbon keeping it as far as possible from your eyes.

  • What do you observe?


    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.

Activity 1.2

  • Take lead nitrate solution in a test tube.

  • Add potassium iodide solution to this.

  • What do you observe?


Activity 1.3

  • Take a few zinc granules in a conical flask or a test tube.

  • Add dilute hydrochloric acid or sulphuric acid to this (Fig. 1.2).

    CAUTION: Handle the acid with care.

  • Do you observe anything happening around the zinc granules?

  • Touch the conical flask or test tube. Is there any change in its temperature?


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


From the above three activities, we can say that any of the following observations helps us to determine whether a 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. We will study about the various types of chemical reactions and their symbolic representation in this Chapter.


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