A drop of acetone, 0.2 g, and a drop of water of the same mass are put on the back of a hand…

Step 1. (a) Acetone. $0.2\ \text{g} = 0.0002\ \text{kg}$, so $Q = 0.0002\ \text{kg} \times 5.2 \times 10^{5}\ \text{J/kg} = 104\ \text{J}$.

Step 2. (a) Water. $Q = 0.0002\ \text{kg} \times 2.26 \times 10^{6}\ \text{J/kg} = 452\ \text{J}$.

Step 3. (b) The acetone takes its $104\ \text{J}$ in a few seconds, so the skin loses heat very fast and feels cold. The water takes its $452\ \text{J}$ slowly, over minutes, and the blood warms the skin again as it goes.