Note on Determination of Latent Heat of Steam by the Method of Mixture

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Measurement of latent heat of vaporization of water
Measurement of latent heat of vaporization of water.

Determination of Latent Heat of Steam by the Method of Mixture

Following are the steps to determine the latent heat of vaporization of water:

  1. A calorimeter with stirrer is taken and it is weighted.
  2. Some water is poured into a calorimeter then mass and temperature of a stirrer, calorimeter and water is taken.
  3. Steam is generated in a steam generator and the temperature of steam is noted.
  4. For a certain time, the steam is passed into the calorimeter containing water.
  5. The final mixture is stir then the temperature and mass of final mixture are taken.

m1= mass of calorimeter and stirrer

m2 = mass of calorimeter, stirrer and water

mw = m1 - m2 = mass of water only

m3 = mass of calorimeter, stirrer, water and steam

m= m3 - m2 = mass of steam only

= temperature of water, stirrer and calorimeter

= temperature of steam

= temperature of mixture

sc = specific heat capacity of a calorimeter

sw = specific heat capacity of water

lv = Latent heat of vaporization = ?

Heat loss by water and calorimeter\( = m l_v +ms_c(\theta_2 -\theta) \)

Heat gain by ice when 0ice and 0water and 0water to water\( = m_ws_w(\theta_1 - \theta) + m_1s_c(\theta_1 - \theta)\)

From principle of calorimetry

\begin{align*} \text {Heat loss} &= \text{Heat gain} \\(m l_v +ms_w(\theta_2 -\theta) &= m_ws_w(\theta_1 - \theta) + m_1s_c (\theta_1 - \theta) \\m l_v &= (m_ws_w + m_1s_c)(\theta - \theta_1)- ms_w(\theta_2 -\theta) \\\therefore l_v &= \frac {(m_ws_w + m_1s_c)(\theta_1 - \theta)- ms_w (\theta_2 -\theta) }{m} \\ \end{align*}

Melting and Freezing

When a solid changes into liquid or liquid changes into solid, the change of state take place. The change of state from solid to liquid is called melting while the change of state from liquid to solid is called freezing. During melting and freezing, the latent heat of fusion is lost to the surroundings while during the melting same amount of heat is absorbed.

Super Cooling

When a liquid is cooled gently, it is found that the liquid may cool even several degrees below its freezing point without solidification. Such a liquid at a temperature below its freezing point is called super cold liquid. Water can be cooled up to the temperature of -20 oCwithout forming ice. But if we drop a small piece of ice into super-cooled water, the water at once will start solidification and immediately the temperature of thewater rises to 0, so this super-cooled state is unstable.

(a) Cooling curve of a typical material (b) Super cooling of a material.
(a) Cooling curve of a typical material (b) Super cooling of a material.

Pressure and Melting Point

At normal temperature, the melting point of a solid or freezing point of a liquid is fixed. But due to increase in pressure, the melting point or freezing point of the substance can change. The melting point can increase or decrease the melting point. In the case of a substance which expands on solidification, the increase of pressure lowers their melting point and in the case of a substance which contracts on solidification the increase of pressure increase their melting point.

Regelation

The phenomenon of melting of ice under the application of pressure and its solidification after the removal of pressure is called regelation. A string can pass through the block of ice without cutting it into two pieces due to regeletion.

Freezing solutions

When a solution is made on dissolving some common salt in water, it lowers the freezing point of water. The higher the quantity of common salt per unit volume of water, the greater is the fall in freezing point. When ice and common salt are mixed, the mixture cools below 0but remains in a liquid. The mixture of salt and ice provides a simple means of reaching a temperature below 0i.e. freezing mixture. This phenomena is important in chemistry especially in metallurgy.

Evaporation

The slow and silent conversion of a liquid into a gaseous state at all temperature is called evaporation. The rate of evaporation depends on following factors:

  1. Area of the liquid surface
  2. Temperature and pressure of the liquid
  3. Nature of the liquid
  4. Effect of wind
  5. Humidity of air

Boiling or Ebullition: It is the rapid and noisy process in which conversion of a liquid into the vapour state takes place. During boiling, the temperature of the liquid remains constant.

Difference between evaporation and boiling

S.N.

Evaporation

S.N.

Boiling

1.

It is the slow and silent process of converting liquid into the vapour state.

1.

It is the rapid and noisy process of converting liquid into water vapour.

2.

It occurs at the surface of the liquid.

2.

It occurs throughout the volume of the liquid.

3.

It takes place at all temperature.

3.

It takes place at a fixed temperature.

4.

Cooling effect is observed.

4.

No cooling is observed.

When a solid changes into liquid or liquid changes into solid, the change of state take place. The change of state from solid to liquid is called melting.

During melting and freezing, the latent heat of fusion is lost to the surroundings while during the melting same amount of heat is absorbed.

The phenomenon of melting of ice under the application of pressure and its solidification after the removal of pressure is called regelation.

When a solution is made on dissolving some common salt in water, it lowers the freezing point of water. 

The slow and silent conversion of a liquid into a gaseous state at all temperature is called evaporation. 

Ebullition is the rapid and noisy process in which conversion of a liquid into the vapour state takes place.

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