Question

How much heat energy is released when $10\text{kg}$ of water at ${100}^{\circ }\text{C}$ is brought to its freezing point?
(Specific heat of water $=1{\text{cal/g}}^{\circ }\text{C}$)

• A. $2100\text{kJ}$
• B. $420\text{kJ}$
• C. $4200\text{kJ}$
• D. $8400\text{kJ}$

Answer 1

The correct option is C $4200\text{kJ}$

We know that,
Freezing point of water is $0^{\circ }\text{C}$
$\therefore$ Change in temperature, $\Delta T=0^{\circ }\text{C}-{100}^{\circ }\text{C}=-{100}^{\circ }\text{C}$
Also,
$Q=mc\Delta T$

Where, $c$ is specific capacity of water
Given: $c=1{\text{cal/g}}^{\circ }\text{C}=1{\text{kcal/kg}}^{\circ }\text{C}$
$=4200{\text{J/kg}}^{\circ }\text{C}$
$\therefore Q=mc\Delta T=10\times 4200\times (-100)$
$=-42\times {10}^5\text{J}$
$=-4200\text{kJ}$
where (-ve) sign indicates that heat is released on cooling.
$\therefore$ Heat released $=4200\text{kJ}$