Heat Transfer Concept Page - 12

Definition

Setup for measurement of latent heat of fusion

A calorimeter is filled with water. A piece of ice is taken and as it is melting, it is dried with a blotting paper and put into the calorimeter. The initial temperature of the water is observed and after all the ice has melted the temperature of the water is taken once again. The weight of the empty calorimeter and weight including water are taken as usual.

Example

Measurement of specific latent heat of ice

Measurement of specific latent heat of ice can be done by:

• Electrical method: Energy is supplied for a known time by an electrical heater of known power.
• Method of mixture: Mixing water and ice in a calorimeter.

Result

Advantages of high specific heat capacity of water

Water is having high heat capacity, its temperature does not change very quickly. Due to this, we observe its various advantages like moderate climate near water bodies (like sea shore), use as a coolant, use as antifreeze, etc.

Result

Change of state

The process of change from one state to another state at a constant temperature is called change of state. It occurs by the exchange of heat. 

Definition

Variation of temperature with time during phase change of a material

Temperature of a material does not change during change of phase. This is because the heat supplied (or released) during the process is used in changing the state of the body.

Definition

Boiling point and normal boiling point

The boiling point of a substance is the temperature at which the vapour pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into a vapour at STP.The normal boiling point (also called the atmospheric boiling point or the atmospheric pressure boiling point) of a liquid is the special case in which the vapour pressure of the liquid equals the defined atmospheric pressure at sea level, 1 atmosphere.

Definition

Factors affecting boiling point

• Pressure: If the external pressure is higher than one atmosphere, the liquid will boil at a higher temperature than the normal boiling point.
• Molecule Types: Types of molecules in the liquid affect the boiling point of the liquid. If the force of attraction between the molecules is relatively strong, the boiling point will be relatively high. If the force of attraction between molecules is relatively weak, the boiling point will be relatively low.

Definition

Experimentally define boiling point of water

Here is the procedure for finding the boiling point of water :
1. Take about 25ml$25ml$ of distilled water in a boiling tube and add 2−3$2-3$ small pieces of pumice stone.
2. Close the mouth of the boiling tube with a rubber cork that has two bores and clamp it with the stand.
3. Introduce a thermometer (temperature range −10  to  110C$-10to110C$) in one bore of the cork of the boiling tube. Keep the bulb of the thermometer  about 3−5cm$3-5cm$ above the surface of the water.
4. Then introduce one end of a delivery tube in the second bore of the cork.Place a 250ml$250ml$ beaker below the second end of the delivery tube to collect the condensed water.
5. Heat the boiling tube gently, preferably by rotating the flame.
6. Note the temperature (t1)$(t_1)$ when the water starts boiling.
7. Continue to heat the water till the temperature becomes constant, and the water remains boiling. Note the constant temperature (t2)$(t_2)$.
8. Record your observations in tabular form t1$t_1$ in one column and t2$t_2$ in another column.Now find the average of t1$t_1$ and t2$t_2$ which will give you the boiling point of water.

Definition

Boiling point and melting point of different substances

The boiling point is the temperature at which a material changes from a liquid to a gas (boils). 
Here are  boiling point of different substances:
pentane (360C)$({36}^{0}C)$, Bromine (58.50C)$({58.5}^{0}C)$, Water (1000C$({100}^{0}C$ etc.
The melting point of a solid is the temperature at which it changes state from solid to liquid at atmospheric pressure.
Here are melting  point of different substances:
mercury (38.830C)$({38.83}^{0}C)$, water (00C)$(0^{0}C)$ Aluminum (6000C)$({600}^{0}C)$ etc.

Definition

Experimentally define melting point of ice

Here is the procedure to determine the melting point of ice:
1. Take some ice cubes and dry them using a filter paper and quickly put them in a beaker.
2. Place the beaker over a wire gauze kept over a tripod stand.
3. Suspend a thermometer (temperature range -10 to 110C) with the help of a clamp stand so that its bulb remains in the middle of the ice cubes.
4. Ice cubes are moderately heated by a gas burner and stirred continuously using a glass rod.
5. Note the temperature (t1)$(t_1)$ when the ice starts melting.Continue heating the ice.
6. Note the temperature (t2)$(t_2)$ when the ice has melted completely.
7. Record your observations in tabular form t1$t_1$ in one column and t2$t_2$ in another column.Now find the average of t1$t_1$ and t2$t_2$ which will give you the melting point of ice.

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