Page 187 - ICSE Chemistry 8

Quick Review

• Water is essen al for the survival and existence of all living organisms.
• The major sources of water are rainwater, surface water and groundwater.
• Rainwater is the purest form of water.
• Surface water consists of water present in oceans, seas, rivers, lakes, etc.
• Water cycle maintains the water balance between land, water bodies and atmosphere. It also controls
weather pa erns and sustains life on the earth.
• Potable water should be clear, transparent, colourless, odourless and free from dissolved and
suspended impuri es.
• The main processes involved in purifi ca on of water obtained from lakes or rivers are sedimenta on,
fi ltra on and chlorina on.
• Purifi ca on of water at home is done by boiling, chlorina on and by using water fi lters and water
purifi ers.
• Pure water is colourless, transparent, odourless and tasteless at ordinary temperature. It is neutral and
highly stable.
• Pure water has a maximum density at 4°C and minimum density at 0°C. The lower density of ice at 0°C
makes it lighter than water and thus, ice fl oats on water.
• Most substances contract when they are cooled. However, water expands when cooled. This is called
anomalous expansion of water.
• Water is called a universal solvent because it dissolves various solids, liquids and gases.
• Hardness of water is caused due to the presence of dissolved chlorides, sulphates and bicarbonates of
calcium and magnesium.
• Water which contains bicarbonates of calcium and magnesium is called temporary hard water as it can
be converted to so water.
• Water which contains chlorides and sulphates of calcium and magnesium is called permanent hard
water.
• Temporary hardness of water can be removed by boiling and permanent hardness of water can be
removed by adding chemicals such as washing soda (sodium carbonate).

EXERCISES

BE PROMPT

A. Fill in the blanks.

1. _____________ is important as it maintains the level of water in the water table.
2. Water purifi ers make water fi t for consump on by using either _________________ or __________
or a combina on of both to kill germs.

3. Water dissolves most _____________ and _____________ that our food contains.
4. The salts containing water of crystallisa on are called _______________.

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