Heat Science: Change of State (part-2)

Freezing mixtures

We know that when a solid dissolve into a liquid, it absorbs heat just as it does on melting. Photographer’s hypo (sodium tho-sulphate), ammonium nitrate, etc. when dissolved in water lower the temperature of water considerably. When a solid dissolves, it takes the necessary amount of heat from the water and consequently cooling occurs.

As the freezing point of a solution is always lower than that of the solvent, the presence of dissolved substances invariably lowers the melting point of a solid or the freezing point of a liquid. If some common salt (Na-Cl) is sprinkled over ice, the temperature falls because ice melts and itself supplies the latent heat needed. The mixture of ice and salt is known as a freezing point. A solution of salt freezes at a temperature much below 0°C. In ice-plants, vats containing fresh water to be frozen, are placed in brine solution which is cooled below 0°C by a form of refrigerator.

Freezing mixtures or cooling agents

Mixture of                                                                   Lowest temperature                     

Ice & salt (3 parts + 1 part)                                                   -22°C        

Ice & calcium chloride (2parts + 3 parts)                            -55°C

Ice & ammonium nitrate (13 parts + 10 parts)                    -17.4°C

Dry ice (solid carbon dioxide)                                              -56°C

Liquid air                                                                                -180°C

Liquid nitrogen                                                                       -195°C

Liquid hydrogen                                                                     -254°C

Liquid helium                                                                          -268°C

Super cooling

Although a solid can never be heated to a temperature higher than its melting point without melting taking place, the reverse is not always true. Many liquids, if allowed to cool undisturbed, may go much below their normal freezing point without solidification taking place. This phenomenon is known as super cooling, or super fusion and the liquid in this condition is said to be super cooled. This super cooled condition is unstable, for if the super cooled liquid is disturbed by stirring or otherwise, solidification at once starts and the temperature quickly rises to the normal freezing point.

Vaporization

The change of a substance from the liquid to the vapor or gaseous state is known as vaporization. It takes place by two distinct processes: (a) evaporation and (b) boiling or ebullition.

In evaporation a liquid is slowly converted to vapor at all temperatures apparently without any supply of heat from outside. In boiling a liquid is rapidly converted to vapor at a definite temperature called boiling point with heat supplied from outside.

Distinction between boiling and evaporation

(i)                 A liquid boils at a definite temperature depending on the super incumbent pressure but evaporates at all temperatures.

(ii)               A liquid is quickly vaporized when it boils but takes a long time when it evaporates-boiling is a more rapid process than evaporation.

(iii)             When a liquid boils, vapor is formed throughout the mass of the liquid; when it evaporates, vapor is formed only on the surface.

Factors that cause rapid evaporation of a liquid

(i)                 The lower boiling point of the liquid.

(ii)               Higher temperature of the liquid; in fact the nearer the temperature to the boiling point the faster is the evaporation.

(iii)             Larger exposed surface.

(iv)              Less super incumbent pressure.

(v)                Lower humidity of air.

(vi)              Removal of vapor in contact with the liquid.

Sublimation and condensation

Sublimation: Sometimes a solid may on heating pass directly into gaseous state without being liquefied. Camphor, iodine, arsenic, sulfur, etc. are examples of such substances. They are called volatile solids. Such a change of state is known as sublimation. Ice and snow also sublime slowly even when below the freezing point.

Condensation: the reverse process of vaporization that is the change from the vapor to the liquid state is known as condensation.

Laws of boiling or ebullition

(i)                 A liquid boils at a definite temperature depending upon the pressure and nature of the liquid. Different liquids boil at different temperatures under normal atmospheric pressure, i.e. 76 cm pressure of mercury. Water boils at 100°C under normal atmospheric pressure.

(ii)               The boiling point of a liquid is raised with the increase of the super incumbent pressure and vice verse.

(iii)             A liquid at its boiling point also needs some heat in order to be converted into vapor without rise of temperature. This heat is absorbed only to bring about the change of state. This is because the kinetic energy of the molecules is the gaseous state is much higher than that in the liquid state. The latent heat is utilized is giving the molecules the excess kinetic energy. The amount of heat required to change 1 gm of a liquid at its boiling point to the vapor state without change of temperature is known as its latent heat of vaporization at that temperature. In the reversed process, the same amount of heat will be given out by 1 gm of the vapor of the liquid during condensation at same temperature.