a) Water of Crystallisation
When crystals of certain salts are formed, they do so with a definite number of molecules of water, chemically combined in a definite proportion. Water of crystallisation is the number of water molecules, chemically combined in a definite molecular proportion, with the salt in the crystalline state. This water is responsible for the geometric shape and colour of the crystals.
Remember:-
A substance containing water of crystallization is called a hydrous substance or a hydrate. This water can be expelled, by heating, and then the salt is said to have become anhydrous.
Some Hydrous Substances
| Name | Chemical formula | Common name |
|---|---|---|
| Zinc sulphate | ZnSO4.7H2O | White vitriol |
| Sodium sulphate | Na2SO4.10H2O | Glauber’s salt |
| Sodium carbonate deca hydrate | Na2CO3.10H2O | Washing soda (crystal) |
| Sodium carbonate deca hydrate | Na2CO3.10H2O | Washing soda (crystal) |
| Potassium aluminium sulphate | K2SO4.Al2 (SO4)3.24H2O | Potash alum |
| Magnesium sulphate | MgSO4.7H2O | Epsom salt |
| Ferrous sulphate | FeS04.7H2O | Green vitriol |
| Copper (II) sulphate | CuSO4.5H2O | Blue vitriol |
| Copper (II) chloride | CuCl2.2H2O | |
| Calcium sulphate semi hydrate | (CaSO4)2.H2O | Plaster of Paris |
| Calcium sulphate | CaSO4.2H2O | Gypsum |
| Calcium nitrate | Ca(NO3)2.4H2O | Lime salt petre or Norwegian salt petre |
| Calcium chloride | CaCl2.6H2O | Dow flake |
| Barium chloride | BaCl2.2H2O |
Crystalline substances without water of crystallization
Certain substances like sodium chloride do not require the help of water to form their crystalline shape.
These salts are:i) Chlorides of potassium, sodium and lead.
ii) Bromides of potassium, sodium, lead and silver.iii) Iodides of silver and lead.
iv) Nitrates of potassium, sodium, lead, silver and ammonia.v) Sulphate of ammonia.
b) Effect of heat on hydrous salt
On heating, hydrous crystals lose their water of crystallization and turn into a powder. They are then said to be anhydrous. Sometimes they may also lose their colour.
Activity
Take hydrated copper (II) sulphate crystals in a test tube and heat. The blue crystals in the test tube start turning white on heating. Water vapours released on heating condense on the cooler part of the test tube. Collect the water drops in a dish. The crystalline copper sulphate turns to a white amorphous form.
Allow the white amorphous powder in the test tube to cool down and then add the water collected in the dish to it; white amorphous powder turns back to blue crystalline form (Fig. 8.6).
c) Efflorescence
It is the property by which certain hydrous crystals when exposed to air, lose their water of crystallisation, fully or partially, and crumble into a powder.
For e.g.,
Such substances are termed as efflorescent substances.
d) Hydroscopic Substances
Hydroscopic substances are those that absorb water vapour from the air. The process is called hydroscopicity.
| Some Hygroscopic substances | |||
|---|---|---|---|
| Concentrated sulphuric acid | H2SO4 | Magnesium chloride | MgCl2 |
| Anhydorus calcium Chloride | CaCl2 | Ferric chloride | FeCl3 |
| Calcium oxide | CaO | Phosphours Pentoxide | P4O10 |
| Solid sodium hydroxide | NaOH | Calcium nitrate | Ca(NO3)2 |
| Solid potassium hydroxide | KOH | Copper nitrate | Cu(NO3) 2 |
e) Deliquescence
Certain hygroscopic substances, not only absorb moisture from the air, but they themselves dissolve in that moisture, undergoing a change of state. Such substances are called deliquescent substances. The process is called deliquescence.
| some Deliquescent substances | |||
|---|---|---|---|
| Anhydrous calcium chloride | CaCl2 | Ferric chloride | Fecl3 |
| Magnesium chloride | MgCl2 | Calcium nitrate | Ca(NO3) 2 |
| Solid sodium hydroxide | NaOH | Copper nitrate | Cu(NO3) 2 |
| Solid potassium hydroxide | KOH | Zinc nitrate | Zn(NO3) 2 |
Drying agents
These are substances that are capable of taking away the moisture from a substance. Since all deliquescent and hydroscopic substances have affinity for water, they can act as drying agents. Usually anhydrous calcium chloride, quick lime and concentrated sulphuric acid are used as drying agents in the laboratory.
f) Action of Dilute Acids on salts
g) Action of Concentrated Sulphuric Acid on Chlorides and Nitrates
It is interesting to note that when metallic nitrates and chlorides are heated with concentrated sulphuric acid, respective acids are formed i.e. nitric acid and hydrochloric acid. This shows less volatile nature of sulphuric acid.
