change in state of matter

There are three states/phases of matter namely solid, liquid, and gas. The same matter can exist in all three phases under different conditions of temperature and pressure. For example, ice (solid) at 0° when heated becomes water (liquid) at 0 °C, which on further heating changes to steam (gas) at 100 °C. Thus at one atmospheric pressure, water is found in all three phases at different temperatures. 

The process of change from one state to another at a constant temperature is called the change of phase. It is brought due to the exchange of heat. 
The change from solid to liquid phase is known as melting, while the reversal change from liquid to solid is called freezing. The change from liquid to vapor is known as vaporization, while the reverse change from gas to liquid is called condensation (or liquefaction). The direct change from solid to vapor is called sublimation and the reverse change from vapor to solid is called deposition. 

MELTING AND FREEZING

The change of solid to liquid phase by the absorption of heat at a constant temperature is called melting. The constant temperature at which a solid changes to liquid is called the melting point of the solid. The reverse change from liquid to solid phase with the liberation of heat at a constant temperature is called freezing and the temperature at which a liquid freezes to solid is called its freezing point. Heat energy is absorbed during melting and it is rejected during freezing at a constant temperature.

The heating curve of ice during melting

Look at the graph above. The temperature of ice remains constant equal to 0 °C in part AB till the whole ice melts. The heat supplied during this time is used to melt the ice. After this, the temperature of water formed by melting ice begins to rise from 0 °C (part BC).

For a pure substance, the melting point and freezing point are identical. For a given mass of substance, the amount of heat energy absorbed during melting is the same as that liberated during freezing. Most substances like lead and wax expand on melting but some substances like ice contract on melting.  The melting point of a substance decreases by the presence of impurities in it. For example, the melting point of ice decreases from 0 °C to -22 °C on mixing salt with it in proper proportion. The melting point of the substances which contract on melting (like ice) decreases with the increase in pressure. On the other hand, the melting point of the substance (such as wax, or lead) which expands on melting increases with the increase in pressure.

VAPORIZATION OR BOILING

The change from liquid to gas (or vapor) phase on the absorption of heat at a constant temperature is called vaporization. The particular temperature at which vaporization occurs is called the boiling point of the liquid. Similarly, the change from vapor to liquid phase on the liberation of heat at a constant temperature is called condensation and the particular temperature at which the condensation occurs is called the condensation point of vapor. 
Heat energy is absorbed at a constant temperature during vaporization, while the same amount of heat energy is liberated during condensation at that temperature for the same mass of the substance. 

The heating curve of water 

At point A, water is at room temperature (20°C) and then with the absorption of heat energy, the temperature of water rises continuously in the part AB where it is in the liquid state. At point B boiling starts and the temperature does not rise further in part BC, the heat energy is continuously absorbed and represents the boiling of water, being B as the boiling point of water. 

For a pure substance, the boiling point and condensation point are identical. The boiling point increases with the increase in pressure and decreases with the decrease in pressure. All liquids expand on boiling.  The boiling point of liquid increases with the addition of impurities to it.

 

Why do we add salt while cooking pulses?  This is based on the fact that adding impurities increases the boiling point of water. We add salt while cooking pulses, the water thus then provides sufficient heat energy to its contents before boiling and so the cooking becomes easier and faster. Why does it take longer to cook food in the hills than in the plains?  This is based on the fact that the boiling point decreases with a decrease in pressure. At high altitudes such as hills or mountains, the atmospheric pressure is low, therefore at these places, water boils at a temperature lower than 100 °C and so it does not provide the required heat energy to its content for cooking. Thus cooking takes a much longer time in such places.

LATENT HEAT AND SPECIFIC LATENT HEAT 

During the change of phase of a substance which takes place at a constant temperature a considerable amount of heat energy is absorbed or liberated. Since the heat energy absorbed or liberated in a change of phase is not externally manifested by any rise or fall in temperature, it is called Latent heat. 
Latent heat, when expressed for a unit mass of a substance, is called specific latent heat and is denoted by the symbol L. 
 

Specific latent heat of a phase is the quantity of heat energy absorbed or liberated by a unit mass of the substance for the change in phase at a constant temperature. If Q amount of heat energy is absorbed (or liberated) by the mass m of a substance during its change of phase at a constant temperature then specific latent heat is  \(\displaystyle L = \frac{Q}{m}\) Therefore, Q the amount of heat energy absorbed or liberated by a given amount of substance for the change of phase whose specific latent heat is L, is Q = mass (m)  × L (specific latent heat)

The SI unit of specific latent heat is J kg^-1, other common units are cal g^-1.
1 cal g^-1 = 4.2 × 10³ J kg^-1

The heat of fusion is the thermal energy that must be withdrawn to solidify a certain mass or quantity of fluid or added to melt a certain mass or quantity of solid. It is also called the latent heat of fusion. Latent heat of vaporization is the heat consumed or discharged when matter disintegrates, changing state from fluid to gas state at a consistent temperature. 
Specific latent heat of fusion of ice is the heat energy required to melt a unit mass of ice at 0 °C to water at 0 °C without any change in temperature. Specific latent heat of freezing of ice is the heat energy liberated/released when a unit mass of water at 0 °C freezes to ice at 0 °C without any change in temperature. For ice, the specific latent heat of fusion is 336000 J kg^-1, which means that 1 kg of ice at 0 °C absorbs 336000 J of heat energy to convert to water at 0 °C. For vaporization, it is the amount of heat (540 cal g⁻¹) expected to change over 1 g of water to 1 g of water fume. A similar measure of heat is released in the stage move during the buildup of 1 g water fume to 1 g of water.

Explanation of latent heat of fusion on the basis of the kinetic model
According to the kinetic model, molecules in a solid vibrate about their mean position. The total energy of a molecule is the sum of the kinetic energy (which depends on the temperature) due to its motion and its potential energy (which depends on the force of attraction between the molecules and the separation between them). When solid changes into liquid without a change in temperature, the average kinetic of the molecules does not change but the separation between the molecules on average increases. Some energy is required to increase the separation against the attractive forces between the molecules( i.e., for the increase in the potential energy of molecules). Thus the heat energy supplied during melting is utilized only in increasing the potential energy of the molecules and is called the latent heat of melting. 
 

Substance	Specific latent heat of fusion in J/g	Specific latent heat of vaporization in J/g
Mercury	11.6	295
Iron	209	6340
Sodium	113	4237
Ice	336	2260

 Examples

• Snow on mountains does not melt all at once: The reason is that ice has a high specific latent heat of fusion (336000 J kg^-1). It changes to water slowly as it gets heat energy from the sun. If latent heat would have been low, all the snow would have melted in a short time on getting heat from the sun and there would be floods in the rivers. 
• In cold countries water in lakes and ponds does not freeze all at once: The reason is that the specific latent heat of fusion of ice is sufficiently high, due to which the water in lakes and ponds will have to liberate a large quantity of heat to the surrounding before freezing. The layer of ice formed over the water's surface being a poor conductor of heat will also prevent the loss of heat from the water of the lake hence it does not freeze all at once.
• Drinks get cooled more quickly by adding a piece of ice than the ice-cold water at 0 °C: This is because 1 gram of ice at 0 °C takes 336 J of heat energy from the drink to melt into the water at 0 °C. This the drinks liberate an additional 336 J of heat energy to 1 gram of ice at 0 °C, than to 1 gram of ice-cold water at 0 °C. 

• Sweating causes cooling: Sweating is a life-saving bodily function that helps regulate your body temperature. When the weather is hot or your body temperature rises due to exercise or fever, sweat is released through ducts in your skin. It contains 90% of water. The sweat drops present on your body utilize the heat of the body for the phase transition from liquid to vapor. This results in a cooling effect that helps to maintain body temperature and cools the body down.
• Always be careful while opening the lid of a pot when the water in it is boiling: Water has a large specific latent heat of vaporization. When steam condenses on the skin of your arm, a very large amount of latent heat is released, which causes serious burns.  

Question 1: How much heat energy is required to melt 10 kg of ice? (Specific latent heat of ice = 336 J g^-1 )
Solution: m = 10 kg, L = 336 J g^-1
Heat energy required = mL = 10000 × 336 = 3360000 J

Question 2: The temperature of 250 grams of water at 40 °C is lowered to 0 °C by adding ice to it. Find the mass of ice added. (Specific latent heat ice is 336 J g^-1  and the specific heat capacity of water is 4.2 J g^-1 K^-1)
Solution: Heat energy lost by water = heat energy gained by the ice
The fall in temperature is 40 − 0 = 40 °C. 
Heat lost by water = m⋅c⋅Δt = 250 × 4.2 × 40 = 42000 J
Heat gained by ice = 42000 = mass of ice × 336 ⇒ mass of ice = 42000 ∕ 336 = 125 g

Question 3: 10125J of heat energy boils off 4.5gms of water at 100°c to steam at 100°c, find the latent heat of steam in S.I. units. 
Solution: Latent heat of steam L = 10125 J ∕ (4.5 × 10 ^-3) kg = 2250 × 10³ J∕kg