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Three liquids with masses $m_1,m_2,m_3$ are thoroughly mixed. If their specific heats are $c_1,c_2,c_3$ and their temperatures $T_1,T_2,T_3$ respectively, then the temperature of the mixture is
$\frac{{{c_1}{T_1}\, +\, {c_2}{T_2} \,+\, {c_3}{T_3}}}{{{m_1}{c_1}\, +\, {m_2}{c_2} \,+\, {m_3}{c_3}}}$
$\frac{{{m_1}{c_1}{T_1}\, +\, {m_2}{c_2}{T_2} \,+ \,{m_3}{c_3}{T_3}}}{{{m_1}{c_1} \,+\, {m_2}{c_2} \,+\, {m_3}{c_3}}}$
$\frac{{{m_1}{c_1}{T_1} \,+\, {m_2}{c_2}{T_2} \,+\, {m_3}{c_3}{T_3}}}{{{m_1}{T_1} \,+\, {m_2}{T_2} \,+\, {m_3}{T_3}}}$
$\frac{{{m_1}{T_1} \,+\, {m_2}{T_2} \,+\, {m_3}{T_3}}}{{{c_1}{T_1} \,+ \,{c_2}{T_2} \,+\, {c_3}{T_3}}}$
Solution
Let the final temperature be $\mathrm{T}^{\circ} \mathrm{C}$
Total heat supplied by the three liquids in coming down to
$0^{\circ} \mathrm{C}$
$=\mathrm{m}_{1} \mathrm{c}_{1} \mathrm{T}_{1}+\mathrm{m}_{2} \mathrm{c}_{2} \mathrm{T}_{2}+\mathrm{m}_{3} \mathrm{c}_{3} \mathrm{T}_{3}$ $…(i)$
Total heat used by three liquids in raising temperature from $0^{\circ} \mathrm{C}$ to $\mathrm{T}^{\circ} \mathrm{C}$
$=\mathrm{m}_{1} \mathrm{c}_{1} \mathrm{T}+\mathrm{m}_{2} \mathrm{c}_{2} \mathrm{T}+\mathrm{m}_{3} \mathrm{c}_{3} \mathrm{T}$ $…(ii)$
By equating $( i )$ and $(ii)$ we get
$\left(\mathrm{m}_{1} \mathrm{c}_{1}+\mathrm{m}_{2} \mathrm{c}_{2}+\mathrm{m}_{3} \mathrm{c}_{3}\right) \mathrm{T}$
$=\mathrm{m}_{1} \mathrm{c}_{1} \mathrm{T}_{1}+\mathrm{m}_{2} \mathrm{c}_{2} \mathrm{T}_{2}+\mathrm{m}_{3} \mathrm{c}_{3} \mathrm{T}_{3}$
$\Rightarrow \mathrm{T}=\frac{\mathrm{m}_{1} \mathrm{c}_{1} \mathrm{T}_{1}+\mathrm{m}_{2} \mathrm{c}_{2} \mathrm{T}_{2}+\mathrm{m}_{3} \mathrm{c}_{3} \mathrm{T}_{3}}{\mathrm{m}_{1} \mathrm{c}_{1}+\mathrm{m}_{2} \mathrm{c}_{2}+\mathrm{m}_{3} \mathrm{c}_{3}}$