Which statement best describes the relationship between fluid temperature and density?

• A. Temperature increases, fluid density increases
• B. Temperature increases, fluid density decreases
• C. Temperature decreases, fluid density decreases
• D. Temperature decreases, fluid density increases

Answer: (B)

When a fluid is heated, its molecules gain kinetic energy and move more vigorously, causing the liquid to expand and occupy a larger volume. Since density is mass per unit volume, the same amount of fluid now has more volume, so its density decreases. This is why the best statement is that increasing temperature leads to decreasing density for most fluids.

In contrast, options that say heating increases density would imply the liquid contracts when heated, which doesn’t align with how most fluids behave. Similarly, cooling typically makes a liquid denser (volume decreases), not less dense. (Note: water has a small anomaly around 4°C, but the general relationship holds for most fluids.)