Melting and Boiling

Md. Saifur Rahman

Melting and Boiling

The process of transforming a solid matter into liquid by means of applying heat is called melting. At normal pressure (1 atmospheric pressure), the temperature at which a solid matter turns into a liquid state is called the melting point of that solid. Each pure solid matter has its own melting point, like the melting point of ice is 0°C.

The process of transforming a liquid matter into a gas by means of heat is called boiling. At normal pressure (1 atmospheric pressure), the temperature at which a liquid matter turns into the gaseous state is called the boiling point of that liquid. Each pure liquid matter has its own boiling point, as the boiling point of water is 100°C.

The boiling process requires heat by solidification requires the removal of heat.

Experiment - 1: Finding the melting point of urea.

Suppose, we want to find out the melting point of urea, a solid matter. First, we need to put a net on a tripod, upon which we shall place a watch glass. Now we shall put some urea on the watch glass. Fix a thermometer with a liner on a stand and put the bulb of the thermometer in the urea. Now start giving heat to the urea. You will find, the urea will start to melt at 133°C temperature and all the fertilizer will melt at the same temperature. This 133°C is the melting point of urea.

Finding the melting point of urea.

Experiment - 2: Finding the melting point of the wax.

Let's find out the melting point of an impure matter, wax. In order to find it out, first, we need to turn the wax into powder. Collect the wax powder inside a glass tube with one side closed. Now fix a thermometer inside following the figure. Sink the glass tube in the beaker in such a way that water can't get inside the tube.

Now, start heating the beaker slowly. You will find the wax meting at a specific range of temperature. This range of temperature is the melting point of the wax.

Finding the melting point of the wax

Melting and Boiling Point Differs Between Pure and Impure substances

The melting point of impure substances is lesser than that of pure substances. It is the opposite in the case with boiling point. A mixed matter will have no fixed melting and boiling point. Since all pure solids have a specific melting point, they melt at that temperature. If a solid substance melts at any other temperature, the solid can be declared impure.

Again, if it melts over a range of temperatures, it is impure too. For example, the melting point of sulfur melts at any other temperature or range of temperatures, the sample of sulfur can be declared adulterated.

Therefore, finding the melting point can be a way to find the purity of solid substances.

Experiment - 3: Boiling Point of Liquids

Some sample of liquid (Example: Water), the boiling point of which needs to be determined, is taken in a beaker will be heated carefully with a Bunsen burner. At a certain temperature, the water in the beaker will turn into vapor. That temperature is the boiling point of water. All the water in the beaker will vaporize at 100°C. That sets the boiling point of water at 100°C (at 1 atm). Since each liquid matter has a specific boiling point, a boiling point cannot be the same for two liquids. Again, if a liquid is adulterated, it will boil at a different temperature. For example, if we add some alcohol to water, it will boil at a different temperature than 100°C.

So, the boiling point can be a determiner of the purity of a liquid.

Boiling Point of water

You have learned, the heat used in boiling and melting does not change the temperature of the matter. It only transforms the state of the matter.

The experiment below will tell you what will happen if we apply heat to a solid to melt it and then heat more to boil it.

Experiment - 4:

Let us take some ice cubes in a beaker and heat them carefully. We shall keep watch on an attached thermometer throughout the process. Let's assume, the initial temperature of the ice was -40°C.

Graph of applying heat on ice

When the temperature of the ice reaches 0°C absorbing the heat, they melt into liquid. The temperature remains stagnant at 0°C until all the ice cubes melt into the water. That probes 0°C as the melting point of ice. The straight line in the temperature is called the melting point line. In the figure, line AB is the melting point line. Both ice and water exist through the length of the line. If we apply more heat, the temperature of water starts to rise. When it reaches 100°C, you'll see more heat will not be able to raise the temperature of water anymore. Rather, the water transforms into vapor. All the water will vaporize at this 100°C temperature. If we apply more heat after that, the temperature of the water vapor will increase. The boiling point of water is 100°C. CD is the line of boiling point. Water and vapor co-exist at this line.

Again, collect the data of cooling the water vapor and set them on a graph sheet with the X-axis telling time and the Y-axis telling temperature. It will be like a figure.

Cooling curve of vapor

The graph shows the initial temperature of water vapor is 140°C. When in the process of cooling it to 100°C, the vapor starts to turn into water. It remains at 100°C until all the vapor is turned into water. Further cooling starts to decrease the temperature of the water. When it reaches 0°C, the water starts to transform into ice. It remains the same until all the water is turned into ice and then the temperature of ice decreases from 0°C.

The figure shows a decrease in the temperature of ice to -40°C.

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