Properties of water

Physical properties

1. Extremely pure water is colorless, odorless, and tasteless.
2. It is a very poor conductor of heat and electricity in its pure form.
3. Pure water freezes at 0°C. It boils at 100°C at a pressure of 760 mmHg, and it boils away completely at this temperature with no change.
4. It has high surface tension compared to many other liquids.
5. It has a high specific heat capacity, meaning it can absorb a large amount of heat before its temperature increases.
6. Water is miscible with many liquids, such as ethanol.
7. The maximum density of pure water is 1 g/cm³ at 4°C. Below this temperature, water begins to expand as it turns into ice.
8. Pure water is neutral to litmus and has a pH of 7. Water expands when it freezes, unlike most other substances.

Chemical properties of water

Action of heat

Water is extremely stable and only decomposes at very high temperatures.

• At about 2500°C, water begins to decompose slightly into hydrogen and oxygen:

$2H_2O(l) \xrightarrow{2500°C} 2H_2(g) + O_2(g)$

• Near 5000°C, water decomposition is almost complete.

Reaction with metals

The reactivity of water with metals depends on the position of the metal in the reactivity (electrochemical) series.

1. Potassium (K): Violent reaction with cold water.

$2K(s) + 2H_2O(l) \rightarrow 2KOH(aq) + H_2(g) \uparrow$

2. Sodium (Na): Vigorous reaction with cold water.

$2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g) \uparrow$

3. Calcium (Ca): Moderate reaction with cold water.

$Ca(s) + 2H_2O(l) \rightarrow Ca(OH)_2(aq) + H_2(g) \uparrow$

4. Magnesium (Mg): Reacts with steam, not cold water.

$Mg(s) + H_2O(g) \xrightarrow{\text{heat}} MgO(s) + H_2(g)$

5. Zinc (Zn): Reacts with steam at high temperature.

$Zn(s) + H_2O(g) \xrightarrow{\text{heat}} ZnO(s) + H_2(g)$

6. Iron (Fe): Reacts with steam at red heat to form iron (II,III) oxide and hydrogen.

$3Fe(s) + 4H_2O(g) \xrightarrow{\text{red heat}} Fe_3O_4(s) + 4H_2(g)$

Reaction with non-metals

1. Carbon (C): Reacts with steam at very high temperature to form water gas (mixture of CO and H₂).

$C(s) + H_2O(g) \xrightarrow{1000°C} CO(g) + H_2(g)$

2. Chlorine (Cl₂): Reacts with water to form a mixture of hydrochloric acid and hypochlorous acid.

$Cl_2(g) + H_2O(l) \rightarrow HCl(aq) + HClO(aq)$

Reaction with oxides

1. With basic oxides (metal oxides): Forms alkaline hydroxides. Example with sodium oxide:

$Na_2O(s) + H_2O(l) \rightarrow 2NaOH(aq)$

Example with calcium oxide:

$CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)$

2. With acidic oxides (non-metal oxides): Forms acids. Example with carbon dioxide:

$CO_2(g) + H_2O(l) \rightarrow H_2CO_3(aq)$

Example with sulfur dioxide:

$SO_2(g) + H_2O(l) \rightarrow H_2SO_3(aq)$