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Chemical Properties Of Water

Pollution of air and water of Class 8

NATURE OF WATER

Water, being a compound of hydrogen and oxygen, is considered on oxide of hydrogen. Since it does not respond to litmus test, it is a neutral oxide.

ACTION OF WATER ON METALS:

Reactions Observations

Potassium:

Potassium reacts with cold water. It even reacts with moisture (water vapour)

2K + 2H2O → 2KOH + H2
  • Potassium floats on water in the form of silvery grey globules.
  • Reaction is vigorous and exothermic.
  • It catches fire and burns with lilac flame.
  • Effervescence of hydrogen is observed and water becomes alkaline.

Sodium:

The reaction takes place in cold water.

2Na + 2H2O → 2NaOH + H2

It even reacts with moisture (water vapour).
  • Sodium floats on water in the form of silvery globules.
  • Reaction is less exothermic an vigorous than that with potassium.
  • It catches fire and burns with golden yellow flame.
  • Effervescence of hydrogen is observed and water become alkaline.

Calcium:

Ca + 2H2O → 2NaOH + H2

The reaction takes place in cold water.
  • Calcium sinks in water and the water becomes milky and alkaline.
  • It does not catch fire.
  • Effervescence of hydrogen is observed.

Magnesium:

The reaction takes place with boiling water, but a moderate reaction takes place when burning magnesium reacts with steam.
  • It burns brilliantly with white light.
  • White ash of MgO is produced.
  • Liberation of hydrogen takes place.

Zinc:

The reaction takes place when steam is passed over red hot zinc.

Zn + H2O → ZnO + H2

(steam)
  • Yellow coloured ZnO is produced. On cooling, yellow coloured ZnO becomes white in colour.
  • Liberation of hydrogen takes place.

Aluminium:

Aluminium also reacts with steam.

2Al + 3H2O → Al2O3 + 3H2

  • A coating of Al2O is formed over the metal which protects the metal from further reaction.
  • Liberation of hydrogen is observed.

Iron:

When steam is passed over red hot iron, the reaction takes place.

3Fe + 4H2O → Fe3O4 + 4H2
  • Brown cloured ferroso ferric oxide is formed and hydrogen is liberated.
  • Reaction is reversible when it takes place in a closed container.

ACTION OF WATER ON NON- METALS:

Reaction with Carbon:

When super heated steam is passed over red-hot coke, a mixture of carbon monoxide and hydrogen, called water gas, is produced. It is a good fuel and reducing agent.

C + H2O → CO + H2 (water gas)

Reaction with Chlorine:

When chlorine gas is passed through water, hydrochloric acid and hypochlorous acid are produced.

Cl2 + H2O → HCl + HOCl.

Reaction with Metallic Oxides

Metallic oxides react with water to form respective bases.

Example:

K2O + H2O → 2KOH

Na2O + H2O → 2NaOH

CaO + H2O → Ca(OH)2

Reaction with non-metallic oxides

Non-metallic oxides react with water to form respective acids.

SO2 + H2O → H2SO3

P2O5 + 3H2O → 2H3PO4

CaO + H2O → Ca (OH)2

Catalytic property:

Water catalyses some chemical reactions.

e.g.

  • The reaction between perfectly dry hydrogen and chlorine does not take place even in the presence of direct sunlight. The reaction is catalysed by a few drops of water.
  • Some other reaction like the reaction between hydrogen and oxygen and the combustion of phosphorous also occur only in the presence of trace amounts of moisture.

2H2 + O2 → 2H2O

4P + O2 → 2P2O5

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