UPSC NDA I 2021 General Ability Solved Paper

Concept:Water has maximum density at $4^\circ\text{C}$, which is $1000\ \text{kg/m}^3$.Explanation:Density is mass per unit volume: $\rho = \frac{m}{V}$.For water, density varies with temperature.At $4^\circ\text{C}$, water reaches its highest density of $1000\ \text{kg/m}^3$ at atmospheric pressure.At $0^\circ\text{C}$, density is slightly lower (about $999.8\ \text{kg/m}^3$), not exactly $1000\ \text{kg/m}^3$.Option A matches the known fact exactly.Options B and C are incorrect because at $0^\circ\text{C}$ density is not $1000$ or $100\ \text{kg/m}^3$.Option D is wrong because $10\ \text{kg/m}^3$ is far too low.When the water molecules are present in ice form then it has a lot of empty space within molecules.When water melts then after converting into the water the molecules get closer to each other and they will be mostly closed to each other at 4°C.Hence the density of water at 4°C is maximum which is 1000 kg/m3.From the above table, it is clear that the density of water at 4°C is 1000 kg/m3. Therefore option 1 is correct.Answer:Option A: Density of water at $4^\circ\text{C}$ is $1000\ \text{kg/m}^3$.