`color{green}(★)` A major part of all living organisms is made up of water.

`color{green}(★)` Human body has about `color{red}(65%)` and some plants have as much as `color{red}(95%)` water.

`color{green}(★)` It is a crucial compound for the survival of all life forms. It is a solvent of great importance.

`color{green}(★)` The distribution of water over the earth’s surface is not uniform.

`color{green}(★)` The estimated world water supply is given in Table 9.2

`color{green}(★)` It is a colourless and tasteless liquid. Its physical properties are given in Table 9.3 along with the physical properties of heavy water.

`color{green}(★)` The unusual properties of water in the condensed phase (liquid and solid states) are due to the presence of extensive hydrogen bonding between water molecules. This leads to high freezing point, high boiling point, high heat of vaporisation and high heat of fusion in comparison to `color{red}(H_2S)` and `color{red}(H_2Se)`.

`color{green}(★)` In comparison to other liquids, water has a higher specific heat, thermal conductivity, surface tension, dipole moment and dielectric constant, etc. These properties allow water to play a key role in the biosphere.

`color{green}(★)` The high heat of vaporisation and heat capacity are responsible for moderation of the climate and body temperature of living beings.

`color{green}(★)` It is an excellent solvent for transportation of ions and molecules required for plant and animal metabolism.

`color{green}(★)` Due to hydrogen bonding with polar molecules, even covalent compounds like alcohol and carbohydrates dissolve in water.

`color{green}(★)` In the gas phase water is a bent molecule with a bond angle of `color{red}(104.5^°)`, and `color{red}(O–H)` bond length of `color{red}(95.7)` pm as shown in Fig 9.1(a).

`color{green}(★)` It is a highly polar molecule, (Fig 9.1(b)).

`color{green}(★)` Its orbital overlap picture is shown in Fig. 9.1(c). In the liquid phase water molecules are associated together by hydrogen bonds.

`color{green}(★)` The crystalline form of water is ice. At atmospheric pressure ice crystallises in the hexagonal form, but at very low temperatures it condenses to cubic form.

`color{green} ✍️ color{green} mathbf("KEY CONCEPT")`

`color{green}(★)` Density of ice is less than that of water. Therefore, an ice cube floats on water. In winter season ice formed on the surface of a lake provides thermal insulation which ensures the survival of the aquatic life. This fact is of great ecological significance.

`color{green}(★)` Ice has a highly ordered three dimensional hydrogen bonded structure as shown in Fig. 9.2.



`color{green}(★)` Examination of ice crystals with X-rays shows that each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of 276 pm.


`color{green}(★)` Hydrogen bonding gives ice a rather open type structure with wide holes. These holes can hold some other molecules of appropriate size interstitially.

`color{green}(★)` Water reacts with a large number of substances. Some of the important reactions are given below.

`color{green}("(𝟏) 𝐀𝐦𝐩𝐡𝐨𝐭𝐞𝐫𝐢𝐜 𝐍𝐚𝐭𝐮𝐫𝐞:")` It has the ability to act as an acid as well as a base i.e., it behaves as an amphoteric substance. In the Brönsted sense it acts as an acid with `color{red}(NH_3)` and a base with `color{red}(H_2S).`

`color{red}(H_2O(l) +NH_3 (aq) → OH^(-) (aq) +NH_4^(+) (aq))`

`color{red}(H_2O(l) +H_2S(aq) → H_3O^(+) (aq) +HS^(-) (aq))`

`color{green}(★ "The auto-protolysis (self-ionization) of water takes place as follows :")`

`color{red}(undersettext{acid-1(acid)}(H_2O(l)) +undersettext{base-2(base)}(H_2O(l)) → undersettext{acid-2(conjugate acid)}(H_3O^(+)(aq)) +undersettext{base-1(conjugate base)}(OH^(-)(aq)))`

`color{green}("(𝟐) 𝐑𝐞𝐝𝐨𝐱 𝐑𝐞𝐚𝐜𝐭𝐢𝐨𝐧𝐬 𝐈𝐧𝐯𝐨𝐥𝐯𝐢𝐧𝐠 𝐖𝐚𝐭𝐞𝐫:")` Water can be easily reduced to dihydrogen by highly electropositive metals.

`color{red}(2H_2O(l) +2Na (s) → 2NaOH(aq) +H_2(g))`

Thus, it is a great source of dihydrogen.

Water is oxidised to `color{red}(O_2)` during photosynthesis

`color{red}(6CO_2(g) +12H_2O(l) → C_6H_(12)O_6 (aq) +6H_2O(l) +6O_2(g))`

With fluorine also it is oxidised to `color{red}(O_2)`

`color{red}(2F_2(g) +2H_2O(l) → 4H^(+)(aq) +4F^(-)(aq) +O_2(g))`

`color{green}("(𝟑) 𝐇𝐲𝐝𝐫𝐨𝐥𝐲𝐬𝐢𝐬 𝐑𝐞𝐚𝐜𝐭𝐢𝐨𝐧:")` Due to high dielectric constant, it has a very strong hydrating tendency. It dissolves many ionic compounds. However, certain covalent and some ionic compounds are hydrolysed in water.

`color{red}(P_4O_(10)(s) +6H_2O(l) → 4H_3PO_4(aq))`

`color{red}(SiCl_4(l) +2H_2O(l) → SiO_2(s) +4HCl(aq))`

`color{red}(N^(3-) (s) +3H_2O(l) → NH_3(g) +3OH^(-) (aq))`

`color{green}("(𝟒) 𝐇𝐲𝐝𝐫𝐚𝐭𝐞𝐬 𝐅𝐨𝐫𝐦𝐚𝐭𝐢𝐨𝐧:")` From aqueous solutions many salts can be crystallised as hydrated salts. Such an association of water is of different types viz.,

(i) coordinated water e.g.,

`color{red}([Cr(H_2O)_6]^(3+) 3 Cl^(-))`

(ii) interstitial water e.g., `color{red}(BaCl_2 . 2H_2O)`

(iii) hydrogen-bonded water e.g., `color{red}([Cu(H_2O)_4]^(2+) SO_4^(2-) .H_2O` in `color{red}(CuSO_4 . 5 H_2O))`