Page 94 - Chemistry ICSE Class IX
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82 ICSE Chemistry – 9
Let us say it Again
Water is essential for life to exist. It is important for humans, animals and plants.
Water is a polar compound and its molecular formula is H O.
2
Water can dissolve different kinds of substances.
Water heats up and cools down slowly because it has a very high specific heat capacity.
Water expands on freezing. Ice is lighter than water. Ice floats on water.
Density of water is maximum at 4°C. This property plays a very important role in nature.
Solvent. The substance in which some other substance is dissolved is called the solvent.
Solute. The substance which is dissolved in any solvent is called solute.
Solution. A homogeneous mixture of two or more substances is called a solution. The solution in which water is the
solvent is called an aqueous solution. The solution in which solute is dissolved in any liquid (other than water) is called
nonaqueous solution.
The solute particles in a true solution are of the order of 10 –10 m.
Unsaturated solution. A solution in which more solute can be dissolved at any fixed temperature is called an unsaturated
solution.
Saturated solution. A solution in which no more solute can be dissolved at any fixed temperature is called saturated
solution.
Supersaturated solution. A solution that holds more solute than its saturated solution can hold at that temperature is
called supersaturated solution.
Solubility. Mass of the solute in grams which dissolves in 100 grams of a solvent to form a saturated solution at any
temperature is called its solubility at that temperature.
Solubility of solid solutes in any liquid, in general, increases with a rise in temperature.
Solubility of a gas in any liquid decreases with a rise in temperature.
Solubility of a gas in any liquid increases with an increase in pressure of the gas on the surface of the liquid.
Solubility curve. A graph between the solubility of a substance and temperature is called its solubility curve.
Water of crystallisation. The water molecules present in the crystal lattice of the substance (per molecule of the
substance) is called its water of crystallisation. Different salts contain different number of water molecules as the water
of crystallisation.
Dissolved oxygen. Water takes up (or dissolves) oxygen from the air until it gets saturated with it. The solubility of
oxygen in water varies from 15 ppm (at 0°C) to 9 ppm (at 20°C). This oxygen dissolved in water is called dissolved oxygen.
Chemical reactions of water. Some typical chemical reactions of water are given below:
Oxides of metals react with water to form alkalis.
Oxides of nonmetals react with water to form acids.
Active metals, such as sodium, potassium and calcium, react with cold water liberating hydrogen gas. Reactions of
water with sodium and potassium are quite vigorous.
Metals, such as zinc and magnesium, react with boiling water to liberate hydrogen gas.
Metals, such as iron, aluminium and nickel, react with steam at higher temperatures to liberate hydrogen.
Reactivity (or activity) series of metals. The arrangement of metals in a vertical column in the order of decreasing
reactivity is called reactivity (or activity) series of metals.
A more electropositive metal displaces less electropositive metal from its salt solution.
More reactive metals occur in the combined form, whereas less reactive metals occur as free metals in nature.
