Matter in Our Surroundings — NCERT Solutions

(a) Water at room temperature is a liquid — Two reasons:

1. Definite volume but no definite shape: Water occupies a definite volume but takes the shape of the container it is kept in, which is a characteristic property of liquids.
2. Fluidity: Water can flow freely from one place to another. The intermolecular forces in water are intermediate — strong enough to keep the molecules together (giving definite volume) but not strong enough to keep them in fixed positions (hence no definite shape). This confirms it is a liquid at room temperature.

(b) An iron almirah is a solid at room temperature — Two reasons:

1. Definite shape and definite volume: An iron almirah has a fixed shape and a fixed volume that does not change on its own, which is a characteristic of solids.
2. Rigidity and incompressibility: Iron is rigid (it cannot be compressed easily) and its particles are held together by very strong intermolecular forces of attraction, keeping them in fixed positions. This confirms it is a solid at room temperature.

Given: Ice at 273 K (0°C) and water at 273 K (0°C).

Concept: Latent heat of fusion.

When ice at 273 K is used for cooling, it first absorbs the latent heat of fusion ($334 \text{ J/g}$ approximately) from the surroundings to melt and convert into water at 273 K. This extra heat absorption (latent heat) makes ice more effective in cooling.

Water at 273 K, on the other hand, does not need to absorb this extra latent heat — it only absorbs heat to raise its own temperature.

Conclusion: Since ice at 273 K absorbs additional latent heat of fusion from the surroundings (in addition to the heat it absorbs to raise its temperature), it produces a greater cooling effect than water at the same temperature (273 K).

Answer: Steam at 100°C produces more severe burns than boiling water at 100°C.

Reason:

Steam at 100°C contains more heat energy than boiling water at 100°C. When steam condenses on the skin, it first releases its latent heat of vaporisation (approximately $2260 \text{ J/g}$) in addition to the heat it gives off while cooling. Boiling water at 100°C only releases heat as it cools down, without releasing any latent heat.

Therefore, steam causes more severe burns because it transfers a much larger amount of heat energy to the skin compared to boiling water at the same temperature.

Note: The diagram shows interconversion of the three states of matter — Solid, Liquid, and Gas — with arrows labelled A, B, C, D, E, and F.

Based on the standard NCERT diagram (Fig. 1.10 type) for Class 9, the labels are as follows:

Summary:

• $A$ = Melting (Solid $\rightarrow$ Liquid)
• $B$ = Evaporation/Vaporisation (Liquid $\rightarrow$ Gas)
• $C$ = Condensation (Gas $\rightarrow$ Liquid)
• $D$ = Freezing (Liquid $\rightarrow$ Solid)
• $E$ = Sublimation (Solid $\rightarrow$ Gas)
• $F$ = Deposition/Reverse Sublimation (Gas $\rightarrow$ Solid)