WATER

Pure water is a colourless, odourless, tasteless, neutral liquid. Pure water does not exist in nature but naturally in varying degree of purity. The main sources of water include rain, springs, borehole, lakes, seas and oceans:

Water is generally used for the following purposes:

1. Drinking by animals and plants.
2. Washing clothes.
3. Bleaching and dyeing.
4. Generating hydroelectric power.
5. Cooling industrial processes.

Water dissolves many substances/solutes. It is therefore called universal solvent. It contains about 35% dissolved Oxygen which support aquatic fauna and flora. Water naturally exists in three phases/states solid ice, liquid water and gaseous water vapour.

The three states of water are naturally inter convertible. The natural inter conversion of the three phases/states of water forms the water cycle. Liquid water in land, lakes, seas and oceans use the solar/sun energy to evaporate/vapourize to form water vapour/gas. Solar/sun energy is also used during transpiration by plants and respiration by animals.

During evaporation, the water vapour rises up the earth’s surface. Temperatures decrease with height above the earth surface increase. Water vapour therefore cools as it rises up. At a height where it is cold enough to below 373Kelvin/100oC Water vapour loses enough energy to form tiny droplets of liquid.

The process by which a gas/water vapour changes to a liquid is called condensation/liquidification. On further cooling, the liquid loses more energy to form ice/solid.

The process by which a liquid/water changes to a ice/solid is called freezing/solidification. Minute/tiny ice/solid particles float in the atmosphere and coalesce/join together to form clouds. When the clouds become too heavy they fall to the earth’s surface as rain/snow as the temperature increase with the fall.

Interconversion of the three phases/stateswater

Pure water has:

1. Fixed/constant/sharp freezing point/melting point of 273K/0oC
2. Fixed/constant/sharp boiling point of 373K/100oC at sea level/1 atmosphere pressure
3. Fixed density of 1gcm-3

This is the criteria of identifying pure/purity of water. Whether a substance is water can be determined by using the following methods:

a) To test for presence of water using anhydrous copper(II)suphate(VI)

Procedure.

1. Put about 2g of anhydrous copper(II)sulphate(VI)crystals into a clean test tube.Add three drops of tap water. Repeat the procedure using distilled water.
2. Observation.
3. Colour changes from white to blue

Explanation.

Anhydrous copper(II)sulphate(VI)is white. On adding water, anhydrous copper(II)sulphate(VI) gains/reacts with water to form hydrated copper(II)sulphate(VI).

Hydrated copper(II)sulphate(VI) is blue. Hydrated copper(II)sulphate(VI) contain water of crystallization.

The change of white anhydrous copper(II)sulphate(VI) to blue hydrated copper(II)sulphate(VI) is a confirmatory test for the presence of water.

Chemical equation.

1. Anhydrous Hydrated
2. copper(II)sulphate(VI) + Water -> copper (II)sulphate(VI)

(white) (blue)

CuSO₄(s) + 5H₂O(l) ->CuSO4.5H₂O(s)

b) To test for presence of water using anhydrous cobalt(II)chloride

Procedure.

Put about 5cm₃ of water into a clean test tube.

Dip a dry anhydrous cobalt(II)chloride paper into the test tube.

Repeat the procedure using distilled water.

Observation.

Colour changes from blue to pink

Explanation.

Anhydrous cobalt(II)chloride is blue. On adding water, anhydrous cobalt(II)chloride gains/reacts with water to form hydrated cobalt(II)chloride. Hydrated cobalt(II)chloride is pink. Hydrated cobalt(II)chloride contain water of crystallization.

He change of blue anhydrous cobalt(II)chlorideto pink hydrated cobalt(II)chloride is a confirmatory test for the presence of water Chemical equation.

Anhydrous Hydrated cobalt(II)chloride + Water ->cobalt (II)chloride (Blue) (pink)

CoCl₂(s) + 5H₂O(l)->CoCl₂.5H₂O(s) Burning a candle in air

Most organic substances/fuels burn in air to produce water. Carbon(IV)oxide gas is also produced if the air is sufficient/excess.

Procedure

Put about 2g of anhydrous copper(II)sulphate(VI)crystals in a boiling tube.

Put about 5cm₃ of lime water in a boiling tube.

Light a small candle stick. Place it below an inverted thistle/filter funnel Collect the products of the burning candle by setting the apparatus as below

Set up of apparatus

Observation

The function pump pulls the products of burning into the inverted funnel. Colour of anhydrous copper(II) sulphate(VI)changes from white to blue.

A white precipitate is formed in the lime water/calcium hydroxide.

Explanation

When a candle burn it forms a water and carbon(IV)oxide.

Water turns anhydrous copper(II) sulphate(VI)changes from white to blue.

Carbon(IV)oxide gas forms white precipitate when bubbled in lime water/calcium hydroxide.

Since:

1. Hydrogen in the wax burn to form water

Hydrogen + Oxygen -> Water

(from candle) (from the air)

2H₂(g) + O₂(g) -> 2H₂O (g/l)

 

2. Carbon in the wax burn to form carbon(IV)oxide

Hydrogen + Oxygen -> Water

(from candle) (from the air)

C(s) + O₂(g) -> CO₂(g)

The candle before burning therefore contained only Carbon and Hydrogenonly. A compound made up of hydrogen and carbon is called Hydrocarbon.

A candle is a hydrocarbon.

Other hydrocarbons include: Petrol, diesel, Kerosene, and Laboratory gas. Hydrocarbons burn in air to form water and carbon(IV)oxide gas.

Hydrocarbons + Oxygen ->Water + Oxygen