Page 184 - ICSE Chemistry 8

HARD WATER AND SOFT WATER

We have learnt that presence of certain salts imparts taste to
drinking water. If soap solu on is added to dis lled water (purest
form of water) and the mixture is shaken, then it forms a good
lather. But presence of certain salts prevent the forma on of lather.
If some soap solu on is added to an aqueous solu on of calcium or
magnesium salt then instead of lather an insoluble substance called
scum is formed.

The water that forms lather easily with soap is called so water. The Fig. 8.9: Scum deposit on a tap
water that does not form lather with soap is called hard water.

Let’s perform Ac vity 8.8 to test the quality of tap water and dis lled water.

Activity 8.8

Aim: To test the quality of tap water and dis lled water
Materials required: A pan, a gas, four cups of water

Procedure
• Boil 4 cups of tap water in a pan for 20–25 minutes.
• Turn off the fl ame and observe carefully.

• Now boil 4 cups of dis lled water in a pan for 20–25 minutes.
• Turn off the fl ame and observe carefully.

Observa on: You will observe that a white layer is formed on the inner surface of the pan in case of tap
water. In case of dis lled water, you will see that no white layer is formed.
Conclusion: The forma on of white layer is due to the presence of impuri es in tap water. This indicates
that tap water is hard water. There are no impuri es present in dis lled water. Therefore, it is so water.

Let’s perform Ac vity 8.9 to observe the forma on of scum in hard water and so water.

Activity 8.9

Aim: To observe the forma on of scum in hard water and so water
Materials required: Two cups of dis lled water, some epsom salt, some liquid detergent, two bo les
Procedure

• Pour 1 cup of dis lled water in a bo le and add a teaspoon of epsom salt to it. Then add a few drops
of liquid detergent. Label it bo le 1.
• Screw the bo le cap ghtly. Shake the bo le vigorously for 10 seconds.

Observa on: In bo le 1, the addi on of epsom salt to dis lled water converts it to hard water. As a result,
you will observe the forma on of scum.

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