ICSE Class 8 Chemistry Chapter 05 Transformation of Substances

Transformation Of Substances

Whenever there is a change in the characteristics of a substance, a chemist wants to know what kind of change has occurred - physical or chemical. You have learnt earlier how to distinguish between physical and chemical changes. Let us recall the following.

Change In State

Change in state is an important physical change. Let us consider the salient features of such a change.

Energy Change In Change In State

Energy change is a general feature of all changes - physical or chemical. So, a change in state is also accompanied by an energy change. This will be evident from the following observations.

1. If you put some ice in a glass, water droplets will collect on the outer surface of the glass. Ice needs heat to melt, which it takes from the surroundings. On giving heat to the ice, the water vapour in the air condenses on the outer surface of the glass.

2. If you have ever used Vicks or Iodex, you would have felt a cooling sensation on your skin. This is because a constituent of the balm evaporates, and takes away the heat required for vaporisation from your skin. The evaporation of a liquid is always accompanied by the cooling of the surface from which the liquid evaporates.

3. Water vapour condenses on a cold surface, making it hot. If some cold water, placed in a pot, is held over the water vapours emanating from boiling water, the cold water becomes warmer. This is because the vapours give out heat while condensing.

Changes In State Occur At Fixed Temperatures

For a pure substance, a change in state occurs at a fixed temperature (provided we do not change the pressure). In all cases, 1 atmosphere is taken as the standard pressure.

A solid melts at a fixed temperature, called its melting point, and a liquid boils at a fixed temperature, called its boiling point. A liquid freezes at its freezing point and a gas (or vapour) liquefies at its temperature of liquefaction. Interestingly, the melting point of a solid is the same as the freezing point of the corresponding liquid. And the boiling point of a liquid is the same as the temperature of liquefaction of the corresponding gas.

The melting point and the boiling point of a pure substance are fixed at a particular pressure.

For example, at a pressure of 1 atmosphere, ice melts at 0°C and water freezes at 0°C, and water boils at 100°C and steam liquefies at 100°C. However, the melting point of a solid decreases and the boiling point of a liquid increases with pressure.

Determining The Melting Point And The Boiling Point Of A Substance

The melting point of ice can be determined by placing the bulb of a thermometer in crushed ice (Figure 5.4a), and recording the temperature when it becomes constant. For other solids, another method (Figure 5.4b) is used. A small amount of the solid is placed in a narrow glass tube, called a capillary tube, which is tied to the bulb of the thermometer. The thermometer, along with the capillary tube, is placed in a suitable bath (a beaker or flask, containing a liquid that boils at a much higher temperature than the melting point of the solid) and heated. The temperature at which the solid melts sharply is recorded.

The boiling point of a liquid is determined using a set-up similar to the one shown in Figure 5.5. A thermometer is introduced into a flask in which the liquid is boiled. The temperature that becomes constant when the liquid keeps boiling is recorded as the boiling point of the liquid.

The Effect Of Impurities On The Melting Point And The Boiling Point Of A Substance

The melting point and the boiling point of a substance change when soluble impurities are present in it.

Melting point

When there are soluble impurities in a substance, its melting point is lowered. For example, sodium chloride is soluble in water, and so the melting point of water (in this case, ice) containing sodium chloride is lower than 0°C at 1 atmosphere. Mix some common salt with ice and record the temperature - it will go to the minus side. This is the principle on which a freezing mixture works. A freezing mixture is a mixture of ice and common salt or calcium chloride, and it helps us attain as low a temperature as -5°, -10° or -15°C, which may be required for an experiment. Ice-cream vendors store ice cream in a freezing mixture.

Boiling point

If there are soluble impurities in a substance, its boiling point is raised.

Boil some water in a beaker and place a thermometer in the boiling water (Figure 5.6). Record the temperature. Dissolve some common salt or sugar in the water, boil it again and record the temperature at which it boils. The water with common salt or sugar dissolved in it boils at a higher temperature than the plain water.

Tap water is not pure water. It contains some dissolved calcium and magnesium salts. So, the boiling point of tap water is also higher than that of distilled water, i.e., pure water. This can be verified by an experiment identical to the one we have just discussed. That tap water contains dissolved impurities can be easily proved as follows. Heat a sample of tap water until all the water evaporates. You will find that a white residue is left behind. Repeat the experiment with distilled water - there will be no residue.

Separating Pure Liquids By Distillation

From a mixture of a pure liquid and a solid (e.g., an aqueous solution of common salt) the liquid can be separated by distillation.

Distillation is performed with a set-up similar to the one shown in Figure 5.7. The mixture is boiled in a flask and the emerging vapours are cooled in a Liebig condenser and collected in a flask. A Liebig condenser is a long tube provided with a jacket, through which water is circulated for cooling the vapours in the tube.

We prepare distilled water in the laboratory by this method.

Fractional distillation

We have touched upon fractional distillation in an earlier chapter. Let us discuss this method here in a little greater detail.

Fractional distillation is used for separating liquids which differ in their boiling points by about 20°C or more. The mixture of liquids is boiled in a flask and the vapours are passed through a fractionating column. The vapours of the liquid with a lower boiling point (i.e., that which is more volatile) rise and the vapours of that which has a higher boiling point (i.e., that which is less volatile) condense in the bubble traps. The higher-boiling liquid may also come back to the flask. The lower-boiling liquid gets into the condenser first and collects in the receiving flask. As long as the lower-boiling liquid continues to distil, the temperature remains constant. After the lower-boiling liquid has distilled off, the higher-boiling liquid begins to distil and the temperature rises. The separately collected liquids are called fractions.

By fractional distillation, we can separate benzene (boiling point 80°C) from toluene (boiling point 110°C), and the different fractions from crude oil.

Teacher's Note

Physical changes like melting and boiling happen around us every day - ice cream melting on a hot day, water boiling for tea, or salt dissolving in cooking. These reversible processes are essential to understanding how matter behaves in our kitchens and the world around us.

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