Permanent Hardness:

Oxide of deuterium ($^2H_1$).
Discovered in 1932 by Urey and Washburn.
Colourless, odourless, tasteless liquid.
Used in preparation of Deuterium Compounds:
- $CaC_2 + D_2O \to CD_2 + Ca(OD)_2$
- $SO_3 + D_2O \to D_2SO_4$
- $Al_4C_3 + 12D_2O \to 3CD_4 + 4Al(OD)_3$
Used as moderator in nuclear reactors.

Reaction of Acid on Zinc Metal:
- Dilute HCl on Zn produces $H_2$ and $ZnCl_2$.
- $Zn + 2HCl \to ZnCl_2 + H_2$
Reaction of a Base on Zinc Metal:
- Aqueous NaOH on Zn produces $H_2$ and sodium zincate ($Na_2ZnO_2$).
- $Zn + 2NaOH \to Na_2ZnO_2 + H_2$

Electrolysis of Water:
- Acidified water using Pt electrodes.
- $2H_2O \to H_2 + O_2$
Electrolysis of Conc. Brine Solution:
- Produces $H_2$ with dichlorine and NaOH.
- $2NaCl + 2H_2O \to 2NaOH + H_2 + Cl_2$
Reaction of Steam over Coke or Hydrocarbon:
- Hydrocarbon or coke with steam at high temp. and catalyst.
- $C_nH_{2n+2} + nH_2O \xrightarrow{1270 K, Ni} nCO + (3n+1)H_2$
- Mixture of $CO + H_2$ is called "Water Gas".

Reaction with Halogens:
- Forms hydrogen halides.
- Order of reactivity: $F_2 > Cl_2 > Br_2 > I_2$
- $H_2 + X_2 \to 2HX$ (where X = F, Cl, Br)
- Example: $H_2 + Cl_2 \to 2HCl$
Reaction with Dioxygen:
- Forms water (highly exothermic).
- $2H_2 + O_2 \xrightarrow{\text{Catalyst or Temp.}} 2H_2O \quad \Delta H = -285.9 KJ mol^{-1}$
Reaction with Dinitrogen:
- Forms ammonia (Haber Process).
- $N_2 + 3H_2 \xrightarrow{673K \text{ or } 200atm} 2NH_3 \quad \Delta H = -92.6 KJ mol^{-1}$
Reaction with Metals:
- Forms metallic hydrides at high temp.
- $2M + H_2 \to 2MH$