ICSE Class 9 Chemistry Chapter 06 Water

Water

Scope Of Syllabus

Water as a compound and as a universal solvent; its physical and chemical properties. Why is water considered a compound? Chief physical properties should include: density, boiling point, melting point. Experiment to show that the water we drink contains dissolved solids and dissolved gases (air): their significance. Solution as 'mixtures' of solids in water; saturated solutions; qualitative effect of temperature on solubility (e.g., solutions of common sulphate, potassium nitrate, sodium chloride in water).

Chemical Properties: The action of cold water on sodium and calcium; the action of hot water on magnesium and steam on iron; reversibility of reaction between iron and steam.

Students can be shown the action of sodium and calcium on water in the laboratory; they must be asked to make observations (equations for the above reactions) and form reactivity series based on reactions.

Important Points To Remember

1. Water is indispensable for the survival of life on the earth. Approximately 80% of the earth's surface is covered by water. Water is the main part of all the living matter.

2. Water exists in all the three states of matter. In all three states water occurs in free form.

(i) Solid: Large amount of water is found in the form of ice or snow.

(ii) Liquid: In liquid state most of the water is present in sea, rivers, lake, ocean, ponds, streams, spring, etc. Water also occurs under the surface of earth called as ground water.

(iii) Gas: Mist, fog are examples of water in its gaseous form. It is also present as water vapours in the atmosphere.

3. In combined state water is present in carbohydrates, proteins, etc. as well as in the salts in the form of water of crystallisation. It is the definite number of water molecules which enters into loose chemical combination when the salt crystallises out of its saturated solution.

For example,

CuSO₄ - 5H₂O - Blue vitriol (Hydrated copper sulphate)

FeSO₄ - 7H₂O - Green vitriol (Hydrated ferrous sulphate)

4. The salts containing water of crystallisation are called hydrated salts.

5. Hydrated salts on heating lose their water of crystallisation and become anhydrous.

6. Hydrated salts on heating lose their crystalline shape and colour. For example, hydrated copper sulphate on heating changes from blue to white and its crystalline shape changes to amorphous.

CuSO₄ - 5H₂O → CuSO₄ + 5H₂O

Blue White

Teacher's Note

Water's ability to exist in three states and dissolve many substances makes it essential for life and explains why it's found everywhere - from ice caps to clouds to ocean water.

7. On heating a hydrated salt, water of crystallisation are given out in the form of steam and they condense on the upper cooler portion of the test-tube.

8. If water is added to an anhydrous compound, the anhydrous compound regains its original colour. For example, if water is allowed to drop over the white anhydrous copper sulphate, then it changes to blue.

9. A glass filled with ice is kept in atmosphere, water droplets condense on the outer cooler surface of glass. This experiment shows that water is present in our atmosphere.

10. In a beaker of 100 c.c, add 50 c.c of water. Over the beaker place the watch glass containing tap water and set the experiment as shown in the figure. Heat the contents of the beaker, the water starts boiling, the steam produced evaporates the water in the watch glass slowly. Continue heating till all the water in the watch glass completely evaporates. The watch glass is removed from the beaker, the concentric rings of solid material are seen on the watch glass. The rings are of salts which are present in dissolved form in water.

11. Water is a compound of hydrogen and oxygen. It can be produced

(i) By burning of hydrogen in air:

Pure hydrogen burns in pure oxygen with pale blue flame to form the droplets of colourless liquid (water).

\[2H_2 + O_2 \longrightarrow 2H_2O\]

(ii) By burning of hydrocarbon in air:

Hydrocarbons burn in free supply of air or oxygen to form carbon dioxide and water vapour.

\[CH_4 + 2O_2 \longrightarrow CO_2 + 2H_2O\]

\[C_2H_4 + 3O_2 \longrightarrow 2CO_2 + 2H_2O\]

\[2C_3H_8 + 5O_2 \longrightarrow 4CO_2 + 2H_2O\]

\[2C_3H_8 + 7O_2 \longrightarrow 4CO_2 + 6H_2O\]

(iii) By the reduction of metallic oxides by hydrogen:

During these reactions, hydrogen gets oxidized to form water.

\[CuO + H_2 \longrightarrow Cu + H_2O\]

\[PbO + H_2 \longrightarrow Pb + H_2O\]

(iv) During the process of respiration:

Carbohydrates burn in the presence of oxygen to form carbon dioxide and water vapour with the liberation of energy.

\[C_6H_{12}O_6 + 6O_2 \longrightarrow 6CO_2 + 6H_2O + Heat\]

Teacher's Note

Water forms whenever hydrogen-containing compounds burn - from hydrogen itself to gasoline in car engines to the carbohydrates we eat for energy, demonstrating water's fundamental role in chemistry and life.

12. The physical properties of water are:

(i) Water is colourless liquid.

(ii) Pure water has flat taste. However, the drinking water has the characteristic taste because of the presence of dissolved salts.

(iii) Boiling point of pure water is 100°C. The constant temperature at which the liquid gets converted into its vapour state is called boiling point. If the pressure increases, then the boiling point also increases. If the pressure decreases the boiling point of water decreases. With an addition of impurity to water the boiling point increases.

(iv) Pure water freezes at 0°C. The constant temperature at which the liquid gets converted into solid is called freezing point. On increasing pressure the freezing point of water decreases. With an addition of impurity to pure water there is depression in its freezing point.

(v) Pure water does not conduct electricity.

(vi) Water shows the anomalous behaviour. All matter expand on heating and contract on cooling, but water behaves abnormally, when heated or cooled between 0°C to 4°C.

(a) When cooled upto 4°C it contracts.

(b) On cooling it further starts expanding instead of contracting and keeps on expanding till 0°C.

(c) Thus at 0°C water has maximum volume but minimum density.

(d) At 4°C pure water has minimum volume and maximum density.

This anomalous behaviour of water is important for the existence of aquatic life in cold countries. As water freezes into ice at 0°C, ice being lighter than water floats on the surface. Water is present below ice where fish and other aquatic animals can easily survive.

(e) Water is a universal solvent as it has the capacity to dissolve the number of solute particles in it. Water is a polar covalent compound. It has a unique property of weakening the electrostatic forces of attraction in ionic compounds thus they rapidly dissolve in water.

(vii) The specific heat capacity of water is 1 calorie/(gram°C). It is the amount of heat (measured in calories) required to raise the temperature of one gram of a substance by one degree celsius.

(viii) The specific latent heat of fusion is the amount of heat needed to convert 1 kg of ice to water at its melting point without change in its temperature. The specific latent heat of fusion for ice is 80 kcal/kg or 333.55 J/g.

(ix) The specific latent heat of vaporization of water is 540 cal/g or 2268 J/g (1 cal = 4.2 J). It is the amount of heat required to change 1g of water to steam at its boiling point without change in its temperature.

13. The solubility of gases in a liquid decreases with rise in temperature and increases with the fall in temperature.

14. On increasing the pressure on the surface of liquid at any temperature, the solubility of gas increases which is in accordance with Henry's law. It states that "At any given temperature, the mass of gas dissolved in fixed volume of liquid is directly proportional to the pressure of the surface of the liquid".

15. The chemical properties of water are

(i) Action towards litmus: Pure water is neutral towards litmus, i.e., it neither turns red litmus to blue nor blue litmus to red.

(ii) Stability: Water is a stable compound. It does not break into its elements when heated in ordinary conditions. However if electric current is passed through acidified water, it

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