WAEC Physics 2024 Paper

Concept:When two samples of the same liquid are mixed in a non-conducting vessel, the heat lost by the hotter liquid is equal to the heat gained by the cooler liquid.Explanation:Let the final temperature of the mixture be $T$.Mass of each sample, $m = 70\,g$.The liquid at $25^\circ\text{C}$ gains heat: $Q_{\text{gained}} = m c (T - 25)$.The liquid at $35^\circ\text{C}$ loses heat: $Q_{\text{lost}} = m c (35 - T)$.Using the principle of heat exchange: $Q_{\text{gained}} = Q_{\text{lost}}$.So, $70 c (T - 25) = 70 c (35 - T)$.Cancelling $70c$ from both sides: $T - 25 = 35 - T$.$2T = 35 + 25 = 60$.$T = \frac{60}{2} = 30^\circ\text{C}$.Answer:The final temperature of the mixture is $30^\circ\text{C}$, i.e., option C.