Matter — Important Questions

Solid

Step 1: In a solid, the inter-molecular force of attraction is very strong. Step 2: The molecules are closely packed and are NOT free to move — they only vibrate about their mean positions. Step 3: Because molecules cannot move freely, the solid maintains its shape — it has a DEFINITE SHAPE. Step 4: Because molecules are tightly packed and cannot move apart, the volume stays constant — it has a DEFINITE VOLUME. Step 5: A liquid has definite volume but NO definite shape; a gas has NEITHER definite shape NOR definite volume.

Inter-molecular space, force of attraction between molecules, and movement of molecules

Step 1: The physical state of any substance is determined at the molecular level. Step 2: The THREE deciding factors are: (i) inter-molecular space — the gap between molecules, (ii) inter-molecular force of attraction — how strongly molecules pull each other, and (iii) movement of molecules — how freely they can move. Step 3: In solids: space is negligible, force is strongest, molecules only vibrate. Step 4: In liquids: space is moderate, force is moderate, molecules move within boundary. Step 5: In gases: space is very large, force is negligible, molecules move freely everywhere.

Liquids have a definite volume but no definite shape

Step 1: The observation shows that water changes its shape in every container — cylindrical, rectangular, and round. This means water has NO definite shape. Step 2: However, the AMOUNT (volume) of water remains the same in all containers. This means water has a DEFINITE VOLUME. Step 3: This behaviour occurs because liquid molecules can move over one another (no fixed positions), allowing shape change. Step 4: But molecules cannot escape the liquid boundary easily (moderate inter-molecular force), so volume is maintained. Step 5: Option C is wrong — liquids do NOT have definite shape. Option D

Gas

Step 1: The inter-molecular force and inter-molecular space are inversely related — as space increases, force decreases. Step 2: In SOLIDS: force is strongest, space is negligible (molecules are tightly packed). Step 3: In LIQUIDS: force is moderate (weaker than solids), space is more than solids. Step 4: In GASES: force is negligible (nearly zero), and inter-molecular space is far larger than in both solids and liquids. Step 5: Because of the negligible force and huge space, gas molecules move freely in all directions, which is why gases have no fixed shape or volume and are highly compressib