Heat - Part II

Change of state and latent heat :In an earlier chapter in Chemistry (Properties of Matter) we have been introduced to how matter changes form when heat is supplied. For example : ice is in a solid state. When heat is applied, it turns into liquid water. When more heat is supplied, water changes into gaseous form that is steam.

The change from solid    liquid    gas is called as a change of state of a substance or a compound.   We know by now that solids are bound together by tight bonds. As we supply energy, the bonds start to stretch. This transforms the solid into liquid state. As more energy is given to the system, the bonds stretch even more and ultimately break. The liquid then is turned into gaseous matter. The inter-molecular forces become weaker as we go from solid to liquid to gaseous state.

Change In State

The process of converting a solid into liquid is known as melting. The temperature at which melting occurs is called the melting point. The inverse of melting is called as freezing or solidification. If solidification makes the solid into properly structured crystals, the process is also known as crystallization process. The process of converting liquid into gas is called vaporizing. The inverse of vaporizing is known as condensation. The temperature at which the liquid turns into gas is called the boiling point of the substance.

Some solids like solid iodine, carbon dioxide, naphthalene balls convert to gaseous state directly from their solid state. They jump the liquid state.  The process of going from solid state to gaseous state directly is known as sublimation. Inverse of sublimation is called as condensation.

We have seen in the earlier section that when heat energy is supplied to a substance, its temperature rises. Take some ice in a beaker. Put a thermometer in the ice. Add a stirrer to make the temperature uniform in the entire mixture. Measure the time and see how the temperature varies. After all the ice has melted into water, heat the water with the help of a lab burner. Heat the water till the water starts boiling.

Plot a graph of Temperature vs Time (in minutes),

                           Graph of Temperature vs Time (in minutes)

You will notice that the ice is at 0ƒC. Even when some of the ice is turned into water, the mixture of ice and water still stays at 0ƒC. The mixture remains at 0ƒC till all the ice has melted.  As the heat is applied, the temperature of the water rises. Then the water starts boiling. The temperature now is 100ƒC. Even if you apply more heat the temperature of the boiling water remains same at 100ƒC. This will happen till all the water is boiled off!! The same observation you will be able to see if you reverse the whole process.

Thus temperature of a substance remains steady during the change of state despite addition (or withdrawal) of energy. The absorption (or release) of heat while changing state is known as the latent heat. 

Latent heat of a substance is the amount of heat absorbed (or released) by a unit mass of substance to change its state without any change in temperature. The MKS unit for latent heat is joules per kilogram or J/kg. Latent heat is denoted by L. The latent heat for solid to liquid is known as the latent heat of fusion. The latent heat from liquid to gas is known as latent heat of vaporization.

If L is the latent heat of a substance and m is the mass of the substance, then the heat Q required (absorbed or released) to change its state = QL

It may be clear now, that the latent heat of ice (to water) is different from the latent heat of steam (water to steam). 

Example 1 :  50 gm of ice at 0ƒC is mixed with 300 gm of water at 70 ƒC. The final temperature of the mixture is 50ƒC. Find the latent heat of ice.

The heat required for the ice to melt into water = Q₁ = mL

                                                                            = 0.05 kg x L (J/kg)

The ice water then has a rise in temperature to 50ƒC.

The heat required to increase the temperature is Q₂ = m x s x T

                                                                            = 0.05 kg x 4180 J/kg/ƒC x (50-0) ƒC

Thus Q₂  = 10450 J

The heat lost by the 300 gm of water to go from 70 ƒC to 50ƒC = Q₃ = m x s x T

                                                                          = .3kg x 4180 J/kg/ƒC x (70-50) ƒC

Thus Q₃  = 25080 J

Heat gained by ice = heat lost by the hot water

Q₁   +   Q₂   =  Q₃

L comes out to be about 3 x 10⁵ J/kg

The actual value of the latent heat of ice is 3.34 x 10⁵ J/kg.

The latent heat of steam is 22.5 10⁵ J/kg.

It is a common knowledge that the burns caused by steam at 100 ƒC is more dangerous than the burns caused by boiling water at 100 ƒC. The reason can be explained if you consider the latent heat. When steam converts into water at 100ƒC, it gives off latent heat of 22. 5 10⁵ J/kg.  This extra energy causes more severe burns. In the same way, you might have heard the term cold ìburnî. This happens when skin is exposed to very cold temperatures. The ìburnî occurs because of the release of latent heat. Take a small piece of ice. Press within your fingers and see how your fingers will burn.

We have seen the definition of melting point and boiling point. These temperatures also depend on the pressure applied. For example, the boiling point reduces if the pressure is lowered. Water boils at about 98.5 ƒC in Pune as it is at an altitude of 500m. The air pressure in Pune is less than the standard at sea level 760 mm of Hg.

Why does food get cooked faster in a pressure cooker? The water in the cooker heats up and becomes steam. The steam contained in the vessel of the cooker, makes the pressure rise. This in turn makes the water boil at higher and higher temperature. The heat thus given to the uncooked food is higher than what it would have got in a normal cooker. The food therefore is cooked faster.

Adding salts can lower the melting and boiling point. If you add common salt to ice, its temperature is lowered to about -23ƒC. Salt water boils at higher temperature than 100ƒC.

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