Physical and Chemical Properties of Water

Water or dihydrogen monoxide is a chemical compound with the  formula H2O.There are two hydrogen atoms covalently bonded to a single oxygen atom in a water molecule.

Water is required in large quantities for agriculture, industrial and household purposes.97.4% of the total water available on the earth is in oceans. So, it is also not available for human consumption. Only 0.01% of the freshwater available in the form of lakes, rivers, underground water etc., is useful.

Properties of water

Water exists in three states they are:

i)Solid state

ii) Liquid state 

iii) Gaseous state.

In solid state, water occurs as snow and frost.When the snow gets compacted, it becomes ice. In liquid state, water occurs as sea water. A very small amount of water only occurs on the land in the form of streams, rivers, lakes etc, which is useful for drinking. Water occurs as water vapor in the air which is the gaseous state of water. 

The properties of water can be studied under two headings-Physical properties and chemical properties.

Physical properties of water

1.Water is a colorless, odorless and tasteless liquid. 

2. The freezing point of water is 0°C at a pressure of 76cm of Hg. However if the pressure increases, the freezing point falls down below 0°C.Freezing point of water decreases due to the presence of impurities.
3. The boiling point of water is 100°C.Pure water boils at 100°C and at a pressure of 76 cm of Hg. Boiling point of water increases when the amount of dissolved impurities increases.
4. The melting point of ice is 0°C. This temperature is constant till all the ice changes into liquid.But here heat energy is stored within the water which is called ‘latent heat of fusion of ice’. Its specific value is 336 joule/gm or 80 cal/gm. Rivers and lakes will not suddenly go frozen because of the high specific heat value.
5. Latent heat of condensation of water is 540 calories/gm. It is the amount of heat energy liberated by converting 1 gm of water vapor at 1000C.
6. When water is heated its temperature starts to increase. ‘Specific heat capacity of water’ can be defined as the amount of heat energy needed to raise the temperature of 1gm of water through 1°C. The specific heat capacity of water is 1cal/gm.0C.Due to high heat capacity of water it is used as coolant.
7. Water is an excellent solvent and therefore it helps in the transportation of ions and molecules required for metabolism in living organisms.
8. On cooling, water starts contracting which means that its volume decreases gradually. This happens till it reaches 4°C. Further decrease in temperature results in increase of volume till it becomes ice at 0°C. As the temperature of water increases from 0°C to 4°C, it contracts. Thus between 0°C to 4°C water on cooling expands and on heating contracts. This is called “Anomalous Expansion of Water”

Chemical properties of water

Some significant reactions of water are as follows:

1. Stability: Water is a highly stable compound. Water does not decompose at ordinary temperatures. On heating water to a very high temperature of about 2000°C, only 0.6% of it gets decomposed to form hydrogen and oxygen.

2H2(g) + O2(g) → 2H2O(l )

2. Nature: Pure water is neutral towards litmus.
3. Catalytic Nature: Water acts as a catalyst in numerous reactions. For example, dry hydrogen and chlorine gasses do not react even in the presence of sunlight unless some traces of water are added to it. 
4. Amphoteric Nature: This represents a capability to perform as a base as well as an acid.

Example:
Acidic Behavior:     H2O(l)+NH3(aq) ⇌ NH4+(aq)+OH−(aq)
Basic Behavior:       H2O(l)+H2S(aq)⇌  H3O+(aq)+HS−(aq)

5. Redox reactions pertaining to H2O

When water reacts with highly electropositive metals like sodium and hydrogen, it can be easily reduced to dihydrogen.

 Example: 2H2O(l)+2K(s)→2KOH(aq)+H2(g)

Water during photosynthesis is oxidized to O2

6.Reactions with metals: Water reacts with active metals like sodium and potassium and forms the corresponding hydroxides.

For example, when a small piece of sodium is dropped into a trough of cold water,it reacts vigorously, catches fire and burns with a golden-yellow flame and hydrogen gas is evolved. The reaction is.

2H2O(l)+2Na(s) ⇌ 2NaOH(s)+ H2(g)+Heat