Question

When a metal bar is cooled, then which one of these statements is correct:
A. Length, density and mass remains same
B. Length decreases, density increases but mass remains the same
C. Length and mass decreases but density remains the same
D. Length and density decreases but mass remains the same

Answer 1

When a metal bar is heated or cooled, the attraction between the molecules either increases or decreases and thus the molecules either come closer or move apart. This causes some variation in the length, mass and density of the metal rod. To find the variation in the density of the rod, use the relation between mass and volume of the rod. Use the above-mentioned basics to answer this question.

Complete step by step answer:
When a metal bar is cooled, the vibration of the molecules decreases due to which the spacing between the molecules decreases. This causes an increase in the attraction between the molecules and the molecules come closer. Thus, the length of the rod decreases. But the quantity of the matter does not change, it remains the same. Hence, the mass remains the same. Density is a ratio of mass to volume. As the length decreases, volume also decreases which leads to an increase in the density of the metal bar.
Hence, when a metal bar is cooled, length decreases, density increases but the mass remains the same.

So, the correct answer is option B i.e. length decreases, density increases but mass remains the same.

Note:
Objects expand in all dimensions that is their area, volume as well as its length increases with increase in temperature. But water is an exception to this rule. When the temperature of water increases after 4°C, it expands and its density decreases. However, the water expands with decreasing temperature when it is between 4°C and 0°C.