Explain Sublimation with the example of Ammonium Chloride or Camphor.

Most substances follow a standard progression: they melt from a solid to a liquid, and then boil into a gas. Sublimation is the "shortcut" in this process.

What is Sublimation?

Sublimation is the process where a substance changes directly from a solid state to a gaseous state (or vice versa) without ever passing through the intermediate liquid state, upon heating.

---

The Ammonium Chloride Experiment

In a laboratory setting, this is most commonly demonstrated using Ammonium Chloride ($NH_4Cl$).

The Setup:

1. Place a small amount of crushed Ammonium Chloride in a china dish.

2. Place an inverted glass funnel over the dish.

3. Plug the narrow opening of the funnel with a bit of cotton to prevent the vapor from escaping.

4. Heat the dish slowly using a burner.

The Observation:

• As the solid Ammonium Chloride heats up, it does not melt into a messy liquid. Instead, it turns directly into white vapors.

• When these vapors rise and touch the cooler, upper walls of the inverted funnel, they lose heat and turn back directly into solid form.

• This secondary process (gas turning back to solid) is often called deposition or solid-desublimation. You will see a crust of pure white solid Ammonium Chloride deposited on the inner neck of the funnel.

---

The Case of Camphor (Kapur)

You may have noticed this in your own home with Camphor or even Naphthalene balls (mothballs):

• Camphor: When you light camphor during a prayer or heat it in a container, it vanishes into the air as a distinct-smelling gas without leaving any liquid residue behind.

• Mothballs: If you place naphthalene balls in a cupboard, they get smaller and smaller over several months. They aren't melting into your clothes; they are sublimating directly into a gas that kills or repels moths.

---

Why does it happen?

Sublimation occurs in substances where the forces of attraction between particles are relatively weak, or where the "vapor pressure" of the solid is high enough that it can jump straight into the air once a little energy is added.