Chemically heavy water is deuterium oxide (`D_2O`). It was discovered by Urey. It has been finding use in nuclear reactors as a moderator because it slows down the fast moving neutrons and therefore, helps in controlling the nuclear fission process.

Water is colourless, odourless and tasteless liquid at ordinary temperature.

Some physical constants of `H _2 O` and `D _2 O` at `298K`.

See Table.

Water shows a versatile chemical behaviour. It behaves as an acid, a base, an oxidant, a reductant and as ligand to metals.

(i) `text(Dissociation of water)` : Water is quite stable and does not dissociate into its elements even at high temperatures. Pure water has a small but measurable electrical conductivity and it dissociates as,

`H_2 O + (H_2O) ⇋ undersettext(Hydronium ion) (H_3O^+) + OH^-` ;

`K_w = 1.0 xx 10 ^-14 mol^2 L^2 ` at `298 K`

(ii) `text(Amphoteric nature)` : Water can act both as an acid and a base and is said to be amphoteric.

However, water is neutral towards litmus and its `pH` is `7`.

(iii) `text(Oxidising and reducing nature)` : Water can act both as an oxidising and a reducing agent in its chemical reactions. e.g.

`2Na + undersettext(Oxidisng Agent) (2 H_2O) -> 2 NaOH +H_2` :

`2F_2 + underset text(reducing Agent) (2H_2O) -> 4HF + O_2`

Water which produces lather with soap solution readily is called soft water. e.g. distilled water, rain water and demineralised water.

Water which does not produce lather with soap solution readily is called hard water. e.g. sea water, river water, well water and tap water.

(i) `text(Cause of hardness of water)` : The hardness of water is due to the presence of bicarbonates, chlorides and sulphates of calcium and magnesium.

Hard water does not produce lather because the cations (`Ca^(+2)` and `Mg^(+2)`) present in hard water react with soap to form insoluble precipitates,

` undersettext[From Hard water] [M^(+2)]` + `undersettext[Sodium stearate (Soap )] [2C_17 H_35 COONa]` `->` `undersettext[Metal stearate ppt] [(C_17 H_35COO)_2M] +2Na^+`

Therefore, no lather is produced until all the calcium and magnesium ions are precipitated. This also results into wastage of lot of soap.

(ii) `text(Type of hardness of water)` : The hardness of water is of two types,

(a) `text(Temporary hardness)` : This is due to the presence of bicarbonates of calcium and magnesium. It is also called carbonate hardness.

(b) `text(Permanent hardness)` : This is due to the presence of chlorides and sulphates of calcium and magnesium. It is also called non- carbonate hardness.

(iii) `text(Softening of water)` : The process of the removal of hardness from water is called softening of water.

It can be removed by the following methods :

(i) `text(By boiling)` : During boiling, the bicarbonates of `Ca` and `Mg` decompose into insoluble carbonates and give `CO_2`. The insoluble carbonates can be removed by filtration.

`undersettext(cal. bicarbonate) [Ca(HCO_3)_2] oversettext(Heat) -> undersettext(Ppt.) (CaCO_3) + CO_2 + H_2O`;

`undersettext(Mag. bicarbonate) [Mg (HCO_3)_2] oversettext(Heat) -> undersettext(Ppt.) (MgCO_3) + CO_2 +H_2O`

(ii) `text(Clark's method)` : This process is used on a commercial scale. In this process, calculated amount of lime [`Ca(OH)_2`] is added to temporary hard water.

`undersettext(Soluble) [Ca(HCO_2)_2] + undersettext(Lime) (Ca(OH)_2) -> undersettext(insoluble) (2CaCO_3) downarrow + 2H_2O`

`undersettext(Soluble) [Mg(HCO_2)_2] + undersettext(Lime) (Ca(OH)_2) -> undersettext(insoluble) (MgCO_3 ) + CaCO_3 downarrow + 2H_2O`

Permanent hardness can be removed by the following methods :

(i) `text(By washing soda method)` : In this method, water is treated with a calculated amount of washing soda (`Na_2CO_3`) which converts the chlorides and sulphates of `Ca` and `Mg` into their respective carbonates which get precipitated.

`CaCl_2 + Na_2CO_3 -> undersettext(Ppt.) (CaCO_3 ) + 2NaCl`;

`MgSO_4 + Na_2CO_3 -> undersettext(Ppt.) (MgCO_3) + Na_2 SO_4`

(ii) `text(Permutit method)` : This is a modern method employed for the softening of hard water. Hydrated sodium aluminium silicate (`Na_2Al_2Si_2O_8. xH_2O`) is called permutit. These complex salts are also known as zeolites.

The permutit as loosely packed in a big tank over a layer of coarse sand. Hard water is introduced into the tank from the top. Water reaches the bottom of the tank and then slowly rises through the permutit layer in the tank. The cations present in hard water are exchanged for sodium ions. Therefore this method is also called ion exchange method.

`undersettext(Sodium zeolite) (Na_2 Z) + undersettext(From Hard water) [Ca^(+2)] -> undersettext(Cal. zeolite) (CaZ) + 2Na^+`;

`undersettext(Sodium zeolite) (Na_2 Z) + undersettext(From Hard water) [Mg^(+2)] -> undersettext(Mag. zeolite) (MgZ) + 2Na^+`

where `Z = Al_2Si_2O_8. xH_2O`

(iii) `text(Calgon)` : The complex salt of meta phosphoric acid, sodium hexa meta phosphate `(NaPO_3)_6`, is known as calgon i.e. `Na_2[Na_4(PO_3)_6]`. Calcium and magnesium salts present in hard water react with calgon to give complex salts.

`2CaSO_4 + Na_2 [Na_4(PO_3)_6] -> Na_2 [Ca_2(PO_3)_6] + 2Na_2SO_4`

`2MgSO_4 +Na_2[Na_4 (PO_3)_6] -> Na_2 [Mg_2 (PO_3)_6 ] + 2Na_2SO_4`

This method is especially used in softening water for boiler use.

(iv) `text(Ion exchange resins)` : Ion exchange resins are the most popular water softeners these days. These resins are synthetic substances. The cation exchanger consists of granular insoluble organic acid resin having giant molecules with `-SO_3H` or `-COOH` groups while the anion exchanger contains giant organic molecules with basic groups derived from amines. Ion exchange resins remove all soluble minerals from water. The hard water is first passed through a bed of cation exchanger, which removes the cations like `Na^+`, `Mg^(+2)`, `Ca^(+2)` and others by exchanging with `H^+` ions.

`undersettext(Resin) (2RH) +Ca^(2+) -> R_2 Ca +2H^+`

The water coming from cation exchanger is acidic on account of free `H^+` ions. This water is then passed through another bed containing anion exchanger. This exchanger removes anions like `Cl^-`, `SO_4^(-2)`, `NO_2^+` by exchanging with `OH^-` ions

`undersettext(Resin) [R - NH_2OH + Cl^(-)] -> R -NH_2Cl + OH^-`

The `OH^-` ions neutralize the `H^+` ions

`H^+ + OH^(-) = H_2O`

`text(Degree of hardness of water)` : The degree of hardness of water is expressed in terms of `text(ppm)` `text[(part per million)]` of `CaCO_3`, i.e.

`text(Degree of Hardness)` = `[text(weight of) CaCO_3]/[text(Weight of) H_2O] xx 10^2`

(i) Water when added to anhydrous copper sulphate, changes its colour from white to blue.

(ii) Water reacts with `CaC_2`, to evolve acetylene which burns with bright flame.