What is latent heat ? Explain with example.
What is latent heat ? Explain with example.
Certain amount of heat energy is transferred between a substance and its surroundings when it undergoes a change of state. The amount of heat per unit mass transferred during change of state of the substance is called latent heat of the substance for the process.
For example, if heat is added to a given quantity of ice at $-10^{\circ} \mathrm{C}$, the temperature of ice increases until it reaches its melting point $\left(0^{\circ} \mathrm{C}\right)$. At this temperature, the addition of more heat does not increase the temperature but causes the ice to melt, or changes its state. Once the entire ice melts, adding more heat will cause the temperature of the water to rise. A similar situation occurs during liquid gas change of state at the boiling point. Adding more heat to boiling water causes vaporisation, without increase in temperature.
The heat required during a change of state depends upon the heat of transformation and the mass of the substance undergoing a change of state.
Thus, if mass $m$ of a substance undergoes a change from one state to the other, then the quantity of heat required is given by
$\mathrm{Q}=m$ L or $\mathrm{L}=\frac{\mathrm{Q}}{m}$
where $\mathrm{L}$ is known as latent heat and is a characteristic of the substance.
Its SI unit is J $\mathrm{kg}^{-1}$.
The value of $\mathrm{L}$ also depends on the pressure. Its value is usually quoted at standard atmospheric pressure.
The latent heat for a solid liquid state change is called the latent heat of fusion ( $\mathrm{L}_{f}$ ), and that for a liquid-gas state change is called the latent heat of vaporisation $\left(\mathrm{L}_{v}\right)$.
These are often referred to as the heat of fusion $\left(\mathrm{L}_{f}\right)$ and the heat of vaporisation.
A plot of temperature versus heat energy for a quantity of water is shown in figure,When heat is added (or removed) during a change of state, the temperature remains constant.
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During summers in India, one of the common practice to keep cool is to make ice balls of crushed ice, dip it in flavoured sugar syrup and sip it. For this a stick is inserted into crushed ice and is squeezed in the palm to make it into the ball. Equivalently in winter, in those areas where it snows, people make snow balls and throw around. Explain the formation of ball out of crushed ice or snow in the light of $p\to T$ diagram of water.
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Triple point of water is
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What is called change of state of matter ?
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