Page 84 - Chemistry ICSE Class IX

72 ICSE Chemistry – 9
Anhydrous Substances

What are anhydrous substances
The substances which are free from water/moisture are called anhydrous
substances.
For example:
Calcium chloride (CaCl )(fused)
2
Copper sulphate (powder) (CuSO )(Anhyd.)
4
Magnesium perchlorate (MgClO )
4
Silica gel (SiO )
2
Table salt (Sodium chloride) (NaCl)

Efflorescence

What is meant by efflorescence
Certain hydrated salts when exposed to dry atmosphere at ordinary
temperature, lose their water of crystallisation and get transformed into
powder. Such salts are called GHƀQTGUEGPVU
The property by which a hydrated salt loses its water of crystallisation
when exposed to dry atmosphere is called GHƀQTGUEGPEG
The ef orescence is favoured by
ƒ Higher atmospheric temperature
ƉƐŽŵ ƐĂůƚ ;DŐ^K ͘ϳ, KͿ ĂŶĚ ƒ Dryness of the air
Ϯ
ϰ
'ůĂƵďĞƌ͛Ɛ ƐĂůƚ ;EĂ ^K ͘ϭϬ, KͿ ĐĂŶ
Ϯ
ϰ
Ϯ
ĂůƐŽ ƵŶĚĞƌŐŽ ĞŋŽƌĞƐĐĞŶĐĞ͘ Examples: Some ef orescent substances are:
ƒ Washing soda (sodium carbonate decahydrate). Washing soda
undergoes ef orescence to give white powder of sodium carbonate
.
monohydrate, (Na CO H O).
2
2
3
dry air
.
.
Na CO 10H O o Na CO H O + 9H O
2
2
3
2
3
2
2
ef orescence
washing soda sodium carbonate
monohydrate
ƒ Blue vitriol (copper sulphate pentahydrate). Blue vitriol undergoes
ef orescence to give monohydrate.
dry air
.
.
CuSO 5H O(s) o CuSO H O(s) + 4H O(g)
2
4
2
4
2
copper sulphate pentahydrate ef orescence copper sulphate monohydrate

Hygroscopic Substances
What are hygroscopic substances
The substances which absorb moisture (water vapour) from the atmosphere
without dissolving in it are called hygroscopic substances. Such a property
is called hygroscopy.
ƒ Hygroscopic substances are generally anhydrous solids or liquids.
ƒ Hygroscopic substances are used as drying agents for drying gases.
Example: Some typical hygroscopic substances are:
Š Anhydrous calcium chloride (CaCl )
2
Š Calcium oxide (or quicklime, CaO)
Š Conc. sulphuric acid (H SO )
4
2