Structure of Water and Ice

Water is a covalent compound. It consists of two hydrogen atoms covalently bonded with the oxygen atom at the centre. In water molecule, the central atom goes SP³ hybridization. According to VSEPR theory, it should have tetrahedral structure but the presence of two lone pair of electron in oxygen increases the repulsion between the H atoms and its shape becomes distorted tetrahedral. The HOH bond angle decreases from 109.5^o to 104.5^o.
Due to the strong electronegative character of oxygen, water molecule is highly polarized. Therefore, there is a formation of intermolecular hydrogen bonding between oxygen of one molecule and hydrogen of another molecule. The extra energy is required to break this molecular bond. Due to this strong association between the hydrogen and oxygen, water molecules become liquid and solid at room temperature. In the absence of hydrogen bond, water would be in gaseous state as that of H₂S. This is the reason for H₂O being liquid at room temperature while H₂S is gas at same temperature while both of the compounds have similar bonding. This is also the reason for anomalous behaviour of water.

Scientist studied that water molecules in ice are arranged in such a way that they form open cage like structure with vacant space due to hydrogen bonding as shown in figure below. With the vacant space, the volume of ice increases. So, as volume has inverse relation with density. Ice has lesser density in compared to water and hence float in water. The figure above shows the arrangement of molecules in ice and water respectively.

Physical and Chemical Properties of Water

Physical properties of water:
– Pure water is transparent, colorless, odourless and tasteless.
– Pure water is a bad conductor of electricity. On addition of impurities, acids, alkalis water can be electrolyzed.
– It has very high dielectric constant and can dissolve most of the ionic compounds.
– It has higher density than ice.
– It is a polar molecule means it has permanent dipole moment.
– It has very high specific heat capacity, latent heat of fusion, latent heat of vaporization etc.
Chemical properties of water:
1. It is neutral in nature. It does not show any reaction with indicators.
2. It is a weak electrolyte due to weak ionization.
3. It reacts with metals to liberate hydrogen.

Zn+H2O−→−−SteamZnO+H2↑

Mg+H2O−→−−SteamMgO+H2↑

2Al+3H2O−→−−SteamAl2O3+3H2↑

4. It reacts with halogens to give oxygen gas.

2H2O+2F2⟶4HF+O2↑

2H2O+2Cl2⟶4HCl+O2↑

5. It reacts with red hot coke to give water gas.

C+H2O⟶CO2+H2OWater gas

6. Water reacts with acidic oxides to give acids and basic oxides to give bases.

SO2+H2O⟶H2SO3Sulphurous acid

CO2+H2O⟶H2CO3Carbonic acid

CaO+HCl⟶CaCl2+H2O

MgO+2HCl⟶MgCl2+H2O

7. It is associated with many salts as water of crystallization such as blue vitriol (CuSO₄.5H₂O), Blue vitriol (FeSO₄.7H₂O) etc.

Hard Water and Soft Water

Water is classified two types on the behaviour upon soap. These are:
1. Soft water: The water that forms lather with soap easily is called soft water. It contain no dissolved salts in it. Rain water, distilled water, dimineralized water are some of the examples of soft water.
2. Hard water: The water that does not forms lather with soap easily is called hard water. It contain dissolved salts ions like SO₄^—, CO₃^—, HCO₃^— of calcium and magnesium in it. Tap water, river water, spring water, sea water are some of the examples of hard water.

Note: Hardness of water is caused by the presence of soluble bicarbonates, chlorides, sulphate of calcium and magnesium. When hard water is added with the potassium and sodium salt of fatty acids from the soap, calcium and magnesium ions of hard water react with soap and forms insoluble curd of fatty acids. So lots of water is wasted and hard water is not considered suitable for washing purposes.

Types of hardness:
– Temporary hardness of water: This type of hardness is caused due to the presence of bicarbonates of calcium and magnesium. This type of hardness is removed by boiling the water.
– Permanent hardness of water: It is due to the presence of chlorides and sulphates of calcium and magnesium. This type of hardness is removed by washing soda process.

Note: In commercial scale, temporary hardness is removed by Clarke’s process.

Both temporary and permanent hardness can be removed by the following processes:
– Soda-lime process
– Calgon process
– Ion exchange method

Heavy Water

Heavy water is also known as deuterium oxide. It was discovered by Urey. It’s molecular formula is D₂O and has molecular weight 20. It can be prepared by the prolonged electrolysis of water.
Some of the uses of heavy water are:
– It is used as a moderator in nuclear reactions. It slow down the nuclear reaction.
– It is used as a tracer in metabolic processes.
– It is also used as a germicides and fungicides.

Note: The rate of reaction of heavy water is slower and decomposes slower than the ordinary water because deuterium bond is much stronger than protium bond.

Chemical properties of heavy water:
1. It decomposes to give oxygen when heated.

2D2O−→Δ2D2+O2

2. Reaction with sodium: It react with sodium to give deuterium molecule.

2D2O+2Na⟶2NaOD+D2

3. Reaction with sulphur trioxide: Heavy water when reacted with sulphur trioxide gives deutero sulphuric acid.

D2O+SO3⟶D2SO4Deutero sulphuric acid.

It is highly toxic in nature so it is not recommended for the living organism. However, the small extent of heavy water is always present in ordinary water. In a tank of 500 litre of ordinary water, the presence of heavy water is about 1 litre.