Q.Give reasons :

1. Conc. HNO$_3$ oxidises Al's surface into a thin, tough Al$_2$O$_3$ film that passivates it against further attack.
2. Al + NaOH liberates H$_2$ and heat, which mechanically/thermally helps clear drain blockages.
3. Graphite's layers, held by weak van der Waals forces, slide over each other — lubricant.
4. Diamond's rigid 3-D covalent network makes it the hardest natural substance — abrasive.
5. Al alloys combine low density with high strength — ideal strength-to-weight ratio for aircraft. …

1. (i) Conc. HNO$_3$ transported in aluminium containers. Concentrated nitric acid, being a strong oxidiser, reacts with the surface of aluminium to form a thin, dense, adherent layer of Al$_2$O$_3$. This oxide film passivates the metal — it prevents the acid from reaching and attacking the metal underneath — so aluminium becomes chemically inert ("passive") to further action of conc. HNO$_3$, making it safe for transport.

2. (ii) Dilute NaOH + aluminium pieces used to open a drain. Aluminium, being amphoteric, reacts with NaOH to liberate hydrogen gas and heat:

The heat evolved helps melt/loosen greasy blockages, while the vigorous evolution of H$_2$ gas creates turbulence that helps dislodge and clear the clog.

3. (iii) Graphite is used as a lubricant. Graphite has a layered structure: within each layer carbon atoms are strongly covalently bonded ($sp^2$, hexagonal sheets), but successive layers are held together only by weak van der Waals forces. These layers can easily slide over one another, giving graphite its characteristic slippery, lubricating property.

4. (iv) Diamond is used as an abrasive. In diamond, every carbon atom is $sp^3$ hybridised and covalently bonded to four others in a rigid, extended three-dimensional network. This gives diamond exceptional hardness (the hardest known natural substance), so it can cut and grind other materials — used in abrasives, cutting tools and drill tips.

5. (v) Aluminium alloys are used for aircraft bodies. Aluminium has low density (light weight), and when alloyed with elements like copper, magnesium or zinc, its strength is greatly enhanced, giving an excellent strength-to-weight ratio — essential for aircraft, where minimising weight without sacrificing structural strength is critical. …

For each part, identify the underlying structural/chemical property responsible: passivation (protective oxide film) for (i) and (vi); amphoteric reaction with alkali for (ii); layered vs 3-D-network bonding of carbon allotropes for (iii)/(iv); density, strength and conductivity of aluminium for (v)/(vii). Stat …

• Saying aluminium "does not react" with conc. HNO$_3$, rather than correctly explaining it is passivated by a protective oxide layer.
• Confusing graphite and diamond's properties (e.g. saying diamond conducts electricity or graphite is used as an abrasive). …