Water
ICSE Class 8 · Chemistry · Chapter 8
What this chapter covers
- Water: a precious resource
- Water as a universal solvent
- Solutions, suspensions and colloids
- Unsaturated, saturated and supersaturated solutions
- Water of crystallisation
- Hygroscopic substances
- Reaction of metals with water and steam
- Hard and soft water
- Disadvantages of hard water
- Removing hardness of water
Key definitions
Dissolution is the process in which the particles of a substance break away from each other and spread evenly between the particles of a liquid, forming a clear mixture.
Solubility is the largest mass of a solute, in grams, that can dissolve in 100 g of a solvent at a given temperature to form a saturated solution.
Water of crystallisation is the fixed number of molecules of water that are chemically joined to one formula unit of a salt in its crystals.
Soft water is water that forms lather easily with soap.
Hard water is water that does not form lather easily with soap, because it contains dissolved salts of calcium and magnesium.
Temporary hardness is hardness caused by dissolved calcium hydrogen carbonate and magnesium hydrogen carbonate, which can be removed by boiling the water.
Permanent hardness is hardness caused by dissolved chlorides and sulphates of calcium and magnesium, which cannot be removed by boiling.
Key points
- Water is essential for life, comes from rain, rivers, lakes, ponds, wells and the sea, and helps control the temperature of its surroundings.
- A solute dissolves in a solvent to form a solution; water dissolves so many substances that it is called the universal solvent.
- True solutions have particles below about 1 nm, colloids between about 1 nm and 100 nm, and suspensions above about 100 nm.
- Only a suspension settles on standing and is held back by filter paper; colloids and suspensions scatter a beam of light.
- An unsaturated solution can dissolve more solute; a saturated one cannot at that temperature; a supersaturated one holds more than a saturated one and is unstable.
- Water of crystallisation is water chemically held in a fixed amount in crystals; hydrated blue CuSO4 · 5H2O becomes anhydrous white CuSO4 on heating.
- Hygroscopic substances such as silica gel, anhydrous calcium chloride, quicklime and concentrated sulphuric acid absorb moisture from the air and are used as drying agents.
- Potassium, sodium and calcium react with cold water to give a hydroxide and hydrogen; magnesium reacts with hot water or steam; zinc and iron react only with steam to give an oxide and hydrogen; copper, silver and gold do not react.
- Hard water contains dissolved calcium and magnesium salts and does not lather easily with soap; soft water lathers easily.
- Hard water wastes soap, forms scum and scale, wastes fuel and harms boilers and pipes.
- Boiling removes temporary hardness; washing soda removes both temporary and permanent hardness.