WAEC Physics 2015 Paper

Concept:The heat lost by the hot water is equal to the heat gained by the cold water.Explanation:Mass of cold water $= 400\text{ g} = 0.4\text{ kg}$.Let the mass of hot water added be $m$ (in kg).Temperature fall of hot water $= 100 - 60 = 40\text{ K}$.Temperature rise of cold water $= 60 - 10 = 50\text{ K}$.Heat lost by hot water $= m \times 4200 \times 40$.Heat gained by cold water $= 0.4 \times 4200 \times 50$.By the principle of calorimetry,$m \times 4200 \times 40 = 0.4 \times 4200 \times 50$.Cancelling $4200$ from both sides gives:$40m = 0.4 \times 50 = 20$.So, $m = \frac{20}{40} = 0.5\text{ kg}$.Converting to grams: $0.5\text{ kg} = 500\text{ g}$.Answer:The mass of hot water added is $500\text{ g}$, which is Option D.