Introduction:

While we commonly observe solids melting into liquids and liquids boiling into gases, nature presents us with some remarkable exceptions where matter takes a direct route, bypassing the liquid state altogether! This fascinating process is known as sublimation.

Sublimation: Solid to Gas, No Liquid In Between!

Sublimation is defined as the direct change of a solid state to gaseous state without changing into liquid state. A classic example readily observed is the behavior of naphthalene balls (mothballs). Over time, these balls seem to "disappear" without leaving any solid or wet residue – they are subliming directly into a gas.

A simple activity (Activity 1.13) can demonstrate this. If you take some camphor, crush it and put it in a china dish. Put an inverted funnel over the china dish and put a cotton plug on the stem of the funnel. Now, heat slowly and observe. You will observe the camphor solid directly transforming into camphor vapours, which then solidify again on the cooler, inner surface of the funnel (a process called deposition).

Deposition: Gas to Solid, The Reverse Direct Path

The opposite process, where a gas changes directly to solid without changing into liquid, is called deposition. This is essentially the reverse of sublimation.

The Power of Pressure: Liquefying Gases and Dry Ice

Beyond temperature, pressure plays an equally crucial role in determining the state of matter. We know that gases are highly compressible as compared to solids and liquids. By applying significant pressure, particles of matter can be brought close together. When combined with a reduction in temperature, this increased pressure can even cause gases to liquefy.

This principle is fundamental to many everyday applications:

• Liquefied Petroleum Gas (LPG): The gas cylinder that we get in our home for cooking is compressed gas (LPG).
• Compressed Natural Gas (CNG): CNG is used as fuel these days in vehicles. Due to its high compressibility, large volumes of a gas can be compressed into a small cylinder and transported easily.

Dry Ice: A Marvel of Pressure and Sublimation

Perhaps the most striking example of both sublimation and the effect of pressure is solid carbon dioxide (CO₂?). It is stored under high pressure. Solid CO₂? gets converted directly into gaseous state on decrease of pressure to 1 atmosphere without coming into liquid state. This is the reason that solid carbon dioxide is also known as dry ice. It never melts into a watery puddle, creating a dramatic fog effect often seen in theatrical productions or for chilling purposes.

Therefore, it's clear that pressure and temperature determine the state of a substance, whether it will be solid, liquid or gas. The interconversion of these states forms a fascinating cycle of transformations within matter.

Fun Fact: Solid CO₂ is called "dry ice" because it converts directly into gaseous state without coming into liquid state, avoiding any wetness.

Question for You: Why is it important that LPG and CNG are stored under high pressure, considering their use as fuels?