Hard water contains salts of calcium and magnesium ions. Hard water does not give lather with soap and forms scum/precipitate with soap. Soap containing sodium stearate $\left(\mathrm{C}_{17} \mathrm{H}_{35} \mathrm{COONa}\right)$ reacts with hard water to precipitate out as $\mathrm{Ca} / \mathrm{Mg}$ stearate.

$$2 \mathrm{C}_{17} \mathrm{H}_{35} \mathrm{COONa}(a q)+\mathrm{M}^{2+}(\mathrm{aq}) \longrightarrow\left(\mathrm{C}_{17} \mathrm{H}_{35} \mathrm{COO}\right)_2 \mathrm{M} \downarrow+2 \mathrm{Na}^{+}(a q)$$ (where, $M$ is $\mathrm{Ca} / \mathrm{Mg}$ )

It is therefore, unsuitable for laundry.

$\mathrm{H}_2 \mathrm{SO}_4$ acts as a catalyst for decomposition of $\mathrm{H}_2 \mathrm{O}_2$. Therefore, some weaker acids such as $\mathrm{H}_3 \mathrm{PO}_4, \mathrm{H}_2 \mathrm{CO}_3$ is preferred over $\mathrm{H}_2 \mathrm{SO}_4$ for preparing $\mathrm{H}_2 \mathrm{O}_2$ from peroxides.

$$3 \mathrm{BaO}_2+2 \mathrm{H}_3 \mathrm{PO}_4 \longrightarrow \underset{\text { (insoluble) }}{\mathrm{Ba}_3\left(\mathrm{PO}_4\right)_2}+3 \mathrm{H}_2 \mathrm{O}_2 $$

In water, oxygen has $s p^3$-hybridisation and the bond angle of HOH should have been $109^{\circ} 28^{\prime}$. In $\mathrm{H}_2 \mathrm{O}$, the oxygen atom is surrounded by two shared pairs and two lone pairs of electrons. From VSEPR theory, lone pair - lone pair repulsions are stronger than bond pair-bond pair repulsions.

As a result, the bond angle of HOH in water slightly decreases from the regular tetrahedral angle of $109^{\circ} .28^{\prime}$ to $104.5^{\circ}$.

Fluorine is a strong oxidising agent, it oxidises $\mathrm{H}_2 \mathrm{O}$ to $\mathrm{O}_2$ or $\mathrm{O}_3$. The reactions are as follows

$$\begin{aligned} 2 \mathrm{~F}_2(g)+2 \mathrm{H}_2 \mathrm{O}(l) \longrightarrow \mathrm{O}_2(g)+4 \mathrm{H}^{+}(a q)+4 \mathrm{~F}^{-}(a q) \\ 3 \mathrm{~F}_2(g)+3 \mathrm{H}_2 \mathrm{O}(l) \longrightarrow \mathrm{O}_3(g)+6 \mathrm{H}^{+}(a q)+6 \mathrm{~F}^{-}(a q) \end{aligned}$$

Water has the ability to act as an acid as well as base, i.e., it behaves as an amphoteric substance. From the Bronsted Lowry theory, it acts as an acid with $\mathrm{NH}_3$ and a base with $\mathrm{H}_2 \mathrm{~S}$.

$\begin{aligned} & \mathrm{H}_2 \mathrm{O}(l)+\mathrm{NH}_3(\mathrm{aq}) \longrightarrow \mathrm{OH}^{-}(\mathrm{aq})+\mathrm{NH}_4^{+}(\mathrm{aq}) \\ & \mathrm{H}_2 \mathrm{O}(l)+\mathrm{H}_2 \mathrm{~S}(\mathrm{aq}) \longrightarrow \mathrm{H}_3 \mathrm{O}^{+}(\mathrm{aq})+\mathrm{HS}^{-}(a q)\end{aligned}$

The auto - protolysis (self-ionisation) of water takes place. The reaction are as follows

$$\underset{\text { Acid }}{\mathrm{H}_2 \mathrm{O}(l)}+\underset{\text { Base }}{\mathrm{H}_2 \mathrm{O}}(l) \longrightarrow \underset{\begin{array}{c} \text { Conjugate } \\ \text { acid } \end{array}}{\mathrm{H}_3 \mathrm{O}^{+}}(a q)+\underset{\begin{array}{c} \text { Conjugate } \\ \text { base } \end{array}}{\mathrm{OH}^{-}(\mathrm{aq})}$$