Matter in Our Surroundings

Formula used: $T(°C) = T(K) - 273$

(a) 293 K to Celsius:
$$T(°C) = 293 - 273 = 20°C$$

(b) 470 K to Celsius:
$$T(°C) = 470 - 273 = 197°C$$

Formula used: $T(K) = T(°C) + 273$

(a) 25°C to Kelvin:
$$T(K) = 25 + 273 = 298 \text{ K}$$

(b) 373°C to Kelvin:
$$T(K) = 373 + 273 = 646 \text{ K}$$

(a) Naphthalene balls disappear with time without leaving any solid:

Naphthalene undergoes sublimation — it directly converts from the solid state to the gaseous (vapour) state without passing through the liquid state. The naphthalene vapours mix with the surrounding air and disperse, which is why no solid residue is left behind.

(b) We can get the smell of perfume sitting several metres away:

Perfume contains volatile liquid substances. These substances evaporate and their particles (molecules) mix with air. Due to the property of diffusion, the particles of perfume spread and intermix with the particles of air and reach our nostrils even from several metres away. Gases diffuse very rapidly because their particles have high kinetic energy and large intermolecular spaces.

Concept: Forces of attraction between particles are weakest in gases, intermediate in liquids, and strongest in solids.

- Oxygen is a gas → weakest intermolecular forces.
- Water is a liquid → intermediate intermolecular forces.
- Sugar is a solid → strongest intermolecular forces.

Increasing order of forces of attraction:
$$\text{Oxygen} < \text{Water} < \text{Sugar}$$

Known facts: Water melts at $0°C$ (273 K) and boils at $100°C$ (373 K) at standard atmospheric pressure.

(a) At 25°C: 25°C is between 0°C and 100°C, so water exists as a liquid.

(b) At 0°C: This is the melting/freezing point of water. At this temperature, water can exist in both solid (ice) and liquid states simultaneously (it is a transition point). Generally, it is considered to be in the solid state (ice) or at the boundary of solid and liquid.

(c) At 100°C: This is the boiling point of water. At this temperature, water can exist in both liquid and gaseous (steam) states simultaneously. It is at the boundary of liquid and gas, generally considered as liquid converting to gas (steam).