Q.(a) Why ice floats on water? [1]

(a) Ice is less dense than water because of its open hydrogen-bonded lattice. (b) H2O2 is stored in black bottles to block light and prevent its photochemical decomposition. (c) Three completed reactions: KMnO4/H2O2 redox, NaNO3 thermal decomposition, and lime water + CO2.

(a) Why ice floats on water:

In liquid water, water molecules are held together by hydrogen bonds, but these are constantly breaking and re-forming, and the molecules are relatively closely packed. When water freezes into ice, each water molecule becomes hydrogen-bonded to four neighbouring water molecules in a fixed, rigid, open (cage-like) tetrahedral three-dimensional lattice. This open hydrogen-bonded structure has more empty space between molecules than liquid water does, making ice LESS dense than liquid water. Since a less dense substance floats on a denser one, ice floats on liquid water (a very unusual property — most solids are denser than their corresponding liquid).

(b) Why hydrogen peroxide is stored in black plastic bottles:

Hydrogen peroxide (H2O2) is thermodynamically unstable and readily undergoes photochemical (light-catalysed) decomposition into water and oxygen:

$2H_2O_2 \xrightarrow{\text{light}} 2H_2O + O_2$

Exposure to light (and to trace metal ions, which can also catalyse this decomposition on ordinary glass surfaces) speeds up this breakdown considerably. To protect it from light and slow this decomposition, H2O2 is stored in dark-coloured (black or brown/amber) wax-lined plastic or glass bottles, which block out light, keeping the peroxide stable for longer.

(c) Completing the reactions:

(i) MnO4^- + H2O2 (in acidic, H+ medium): This is a redox reaction where H2O2 acts as a reducing agent (getting oxidised to O2) and MnO4^- (purple) acts as the oxidising agent (getting reduced to pale pink/colourless Mn2+):

$2MnO_4^- + 5H_2O_2 + 6H^+ \rightarrow 2Mn^{2+} + 5O_2 + 8H_2O$

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