a) Anhydrous `H_2O_2` is colourless, viscous liquid soluble in ether, alcohol and water. It gives blue tinge in thick layers.

b) It causes blisters on skin and bitter in taste.

c) Pure `H_2O_2` is weak acidic in nature and exist as associated liquid due to hydrogen bonding.

d) Its specific gravity is `1.45` `g//ml` at `0^oC`.

e) It is diamagnetic in nature.

f) It boils at `152^o`C and freezes at `0.89^oC`. It begins to decompose at b. pt. and thus distilled under reduced pressure.

g) In pure state, its dielectric constant is `93.7` which increases with dilution. Due to high dielectric constant, `H_2O_2` and its aqueous solution are good solvent.

(i) `text(Decomposition)` : Pure `H_2O_2`, is an unstable liquid and decomposes into water and `O_2`, either upon standing or upon heating,

`2H_2O_2 -> 2H_2O + O_2`; `DeltaH = -196.0 kJ`

(ii) `text(Oxidising nature)` : `H_2O_2`, is a powerful oxidant in acidic as well as in alkaline medium.

(`text(In acidic)`) `H_2O_2 + 2H^+ + 2e^(-) -> 2H_2O` slow
(`text(In alkali)`) `H_2O_2 + 2e^(-) -> 2OH^-` fast

Thus `H_2O_2`, is more powerful oxidant in alkaline medium. The simple interpretation of `H_2O_2`, as oxidant can be shown by the equation

`H_2O_2 -> H_2O +O`

Following are some important examples of oxidant action of `H_2O_2`

(a) `H_2O_2` oxidises black lead sulphide (`PbS`) to white lead sulphate (`PbSO_4`)

`PbS + 4H_2O_2 -> PbSO_4 + 4H_2O`

(b) `H_2O_2` oxidizes sulphites into sulphates

`Na_2SO_3 + H_2O_2 -> Na_2SO_4 + 4H_2O`

(c) `H_2O_2` oxidizes nitrites to nitrates

`2NaNO_2 + H_2O_2 -> NaNO_3 + H_2O`

(d) `H_2O_2` Iiilberates iodine from potassium iodide

`2KI + H_2O_2 -> 2KOH + I_2`

(e) `H_2O_2` oxidizes arsenates into arsenates

`Na_2 AsO_3 + H_2O_2-> Na_3AsO_4 + H_2O`

(f) `H_2O_2` oxidizes acidified ferrous sulphate to ferric sulphate

`2FeSO_4 + H_2SO_4 +H_2O_2-> Fe_2(SO_4)_2 + 2H_2O`

(g) `H_2O_2`, oxidizes `H_2S` into sulphur

`H_2S + H_2O_2 -> 2H_2O + S`

(h) `H_2O_2`, oxidizes acidified potassium ferrocyanide to potassium ferricyanide

`2K_4Fe(CN)_6 + H_2SO_4 + H_2O_2 -> 2K_3 Fe(CN)_6 + K_2SO_4 +H_2O`

(i) `H_2O_2`, oxidizes formaldehyde to forinic acid in presence of pyrogallol and in alkaline medium

`2HCHO +H_2O_2 -> 2HCOOH+H_2`

(j) `H_2O_2`, dissolves the chromic hydroxide precipitate present in `NaOH` solution forming a yellow solution of sodium chromate

`2Cr(OH)_3 +4NaOH + 3H_2O_2 -> 2Na_2CrO_4 + 8H_2O`

(k) `H_2O_2`, oxidizes Benzene in presence of ferrous sulphate to phenol

`C_6H_6 + H_2O_2 overset(FeSO_4)-> C_6H_5OH`

(l) A solution of chromic acid in sulphuric acid or acidified potassium dichromate is oxidized to blue peroxide of chromium `(CrO_5)` which is unstable, however, it is soluble in ether and produced blue coloured solution

`K_2Cr_2O_7 +H_2SO_4 + 4H_2O_2 -> 2CrO_3 + K_2SO_4 + +5 H_2O`

Peroxide of chromium decomposes to form chromic sulphate & oxygen

`4CrO_5 + 6H_2SO_4 -> 2Cr_2 (SO_4)_3 + 6H_2O + 7O_2`

(m) `H_2O_2` oxidizes mercury to mercuric oxide in acid solution

`Hg + H_2O_2 overset (H_2SO_4) (->) HgO + H_2O`

(iii) `text(Reducing nature)` : It can also act as a reducing agent towards powerful oxidising agents

`H_2O_2 -> 2H^+ + O_2 + 2e^-`

In alkaline solution, however, its reducing action is more effective

`H_2O_2 + 2OH^(-) ->2H_2O + O_2 + 2e^-`

The simple equation when `H_2O_2` acts as a reducing agent can be expressed as,

`H_2O_2 + O ->H_2O +O_2`

(a) It reduces `Ag_2O` to silver

`Ag_2O +O_2 -> 2Ag + H_2O + O_2`

(b) It reduces ozone to oxygen

`H_2O_2 +O_2 -> H_2O +2O_2`

(c) It reduces manganese dioxide in acidic medium to manganese sulphate

`MnO_2 + H_2SO_4 + H_2O_2 -> MnSO_4 + 2H_2O +O_2`

(d) It reduces lead dioxide to lead monoxide

`PbO_2 + H_2O_2 -> PbO + H_2O + O_2`

(e) It reduces red lead in presence of `HNO_2`, to plumbous salt

`Pb_3O_4 + 6HNO_3 + H_2O_2 -> 3Pb(NO_3)_2 + 4H_2O +O_2`

(f) Chlorine and bromine are reduced to corresponding hydracids

`Cl_2 +H_2O_2 -> 2HCl +O_2`

The reaction can be shown in following steps:

`Cl_2 +H_2O ->HCl +HClO`
`ul(HClO +H_2O_2 -> HCl +H_2O +O_2)`
`Cl_2 + H_2O_2->2HCl +O_2`

Similarly `Br_2 +H_2O_2 -> 2HBr +O_2`

(g) It reduces acidified `KMnO_4` solution i.e., acidified `KMnO_4` is decolourised by `H_2O_2`

`2KMnO_4 + 3H_2SO_4 + 5H_2O_2 -> K_2SO_4 +2MnSO_4 + 8H_2O +5O_2`

(h) It reduces potassium ferricyanide (alk. solution) to potassium ferrocyanide

`2K_3Fe(CN)_6 +2KOH +H_2O_2 ->2K_4Fe(CN)_6 + 2H_2O +O_2`

(i) It reduces hypohalites to halides

`NaOCl +H_2O_2 -> NaCl + H_2O + O_2`

(iv) `text(Bleaching action)` : `H_2O_2` acts as a bleaching agent due to the release of nascent oxygen.

`H_2O_2 -> H_2O +O`

Thus, the bleaching action of `H_2O_2` is due to oxidation. It oxidises the colouring matter to a colourless product, Colouring matter `+O` `->` Colourless matter. `H_2O_2` is used to bleach delicate materials like ivory, silk, wool, cotton, hair, leather etc.

(v) `text(Addition reations)` : It shows addition reactions on ethylenic bond

`underset (CH_2) overset (CH_2) (||) + H_2O_2 ->CH_2(OH)-CH_2OH`

`H_2O_2` has non-planar structure in which two `H`-atoms are arranged in two directions almost perpendicular to each other and to the axis joining the two oxygen atoms. The `O-O` linkage is called peroxide linkage. See fig.

The strength of `H_2O_2` is expressed in terms of weight or volume,

(i) `text(As weight percentage)` : The weight percentage of `H_2O_2`, gives the weight of `H_2O_2`, in `100` g of solution.

For example, a `40%` solution by wt. means `40` g of `H_2O_2`, are present in `100` g of solution.

(ii) `text(As volume)` : The strength of `H_2O_2` is commonly expressed as volume. This refers to the volume of oxygen which a solution of `H_2O_2` will give. For example, a `20` volume of `H_2O_2`, means that `1` litre of this solution will give `20` litres of oxygen at `NTP`.

(i) As germicide and antiseptic for wounds, teeth and ears.

(ii) As bleaching agent for wood, hair and other soft materials.

(iii) As preservative for milk and wine.

(iv) As fuel for rocket, submarine and torpedo.

(v) In refreshing old oil paintings due to the formation of black `PbS`. `H_2O_2` converts it into white `PbSO_4`

`undersettext(black) (PbS) + 4H_2O_2 -> underset (white)(PbSO_4) + 4H_2O`

(vi) In the detection of `Ti^(+4)`, `V^(+5)` and `Cr^(+3)` ions.

(vii) As antichlor to remove traces of chlorine and hypochlorite.

(i) An acidified solution of titanium salt when treated with `H_2O_2`, a yellow or orange colour is developed due to formation of pertitanic acid

`Ti(SO_4)_2 + H_2O_2 + 2H_2O -> H_2TiO_4 + 2H_2SO_4`

(ii) It liberates iodine from `KI` solution which gives blue colour with starch solution.

(iii) It decolourizes acidified `KMnO_4` solution.

(iv) `H_2O_2` on shaking with acidified `K_2Cr_2O_2` with little ether, blue colour is produced (due to formation of peroxide of chromium).

(v) A filter paper with a black stain of `PbS` on bringing in contact with `H_2O_2` solution turns white.