Page 181 - ICSE Chemistry 8

Activity 8.7

Aim: To show that copper sulphate crystals contain water of crystallisa on
Materials required: Test tube, blue copper sulphate crystals, test tube holder, a burner
Procedure

• Take a dry test tube and fi ll it with some blue copper sulphate
crystals.
• Hold the test tube using a test tube holder in such a way that drops of
the drops of water may not slip back and crack the test tube at water
its hot end.

• Heat the test tube carefully.
blue vitriol
Observa on: You will observe that drops of a colourless liquid get
condensed on the upper cooler parts of the test tube and the blue
burner
crystalline solid of copper sulphate turns into a white amorphous
(powder) solid. This happens due to the loss of water molecules
from the crystals.
• Cool the test tube and let the condensed colourless liquid drops trickle back to the white solid.

Observa on: A er cooling, you will observe that the solid again changes into blue. Therefore, the
colourless liquid is water.
Conclusion: This shows that copper sulphate crystals contain water of crystallisa on. The salt gets its blue
colour because of this reason.

The salts containing water of crystallisa on are called hydrated salts. When heated, these salts lose the
water of crystallisa on and are called anhydrous salts.
There are certain substances that absorb moisture from air but the amount is not suffi cient to form a
solu on. Such substances are called hygroscopic substances. Some hygroscopic substances are calcium
oxide (CaO), calcium chloride (CaCl ), phosphorus pentoxide (P O ), concentrated sulphuric acid (conc.
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H SO ), concentrated hydrochloric acid (conc.HCl) and silica gel. All the anhydrous salts such as anhydrous
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copper sulphate, anhydrous ferrous sulphate and anhydrous sodium carbonate are also hygroscopic
substances.
Hygroscopic substances are used as drying agents to remove moisture from other substances without
reac ng with them chemically. Due to this reason, small silica gel packets are kept in bo les to keep them
dry as they absorb moisture.

Hygroscopy refers to the process of absorbing moisture from air without transforming the state of a
substance.
At mes, a hygroscopic substance may absorb the required amount of moisture and form an aqueous
solu on. This phenomenon is known as deliquescence and the substance exhibi ng the phenomenon
.
is called deliquescent substance. CaCl , CaCl 6H O, FeCl , KOH, P O and MgCl are some examples of
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deliquescent substances. For example, drying tubes are frequently packed with CaCl for drying the
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