MCQ
$1\, g$ of a steam at $100°C$ melt ........ $gm$ ice at $0°C\,?$ $($Latent heat of ice $= 80 \,cal/gm$ and latent heat of steam $= 540\, cal/gm)$
- A$1$
- B$2$
- C$4$
- ✓$8$
✓
Answer
Correct option: D.
$8$
d
(d) Suppose $m\, gm$ ice melted, then heat required for its melting
(d) Suppose $m\, gm$ ice melted, then heat required for its melting
$ = mL = m \times 80\,cal$
Heat available with steam for being condensed and then brought to $0°C$
$ = 1 \times 540 + 1 \times 1 \times (100 - 0) = 640\,cal$
$\Rightarrow$ Heat lost $=$ Heat taken
$\Rightarrow$ $640 = m \times 80$ $\Rightarrow$ $m = 8\,gm$
Short trick: You can remember that amount of steam $(m')$ at $100°C$ required to melt m gm ice at $0°C$ is $m' = \frac{m}{8}$.
Here, $m = 8 \times m' = 8 \times 1 = 8\,gm$
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