2.6 Melting and Boiling

Melting:

• Melting is the process of converting a solid to a liquid by applying heat.

• At normal pressure (1 atmosphere), the temperature at which a solid turns into a liquid is called the melting point.

• Example: The melting point of ice is 0°C.

Boiling:

• Boiling is the process of converting a liquid into a gas by applying heat.

• At normal pressure (1 atmosphere), the temperature at which a liquid turns into a gas is called the boiling point.

• Example: The boiling point of water is 100°C.

Differences Between Melting and Boiling:

• Melting requires heat but solidification requires the removal of heat.

• Boiling always happens at a specific temperature (boiling point) depending on the substance, while melting can be observed at different temperatures for pure and impure substances.

Experiments:

Experiment 4: Finding the Melting Point of Urea

• To find the melting point of urea, heat a sample of urea in a watch glass placed on a tripod.

• Fix a thermometer in the urea and heat it until it melts at 133°C. This temperature is the melting point of urea.

• For impure substances like wax, the melting point is not fixed and is a range of temperatures, as seen when wax melts over a range of temperatures.

Experiment 5: Boiling Point of Liquids

• To find the boiling point of water, place water in a beaker and attach a thermometer.

• Heat the water with a Bunsen burner. When the water starts turning into vapor, this indicates the boiling point of water at 100°C.

• If any impurities (like alcohol) are added to the liquid, the boiling point changes, thus boiling point can also determine the purity of a liquid.

Experiment 6: Heating Ice

• Take ice cubes in a beaker and attach a thermometer.

• As the ice absorbs heat, its temperature increases until it reaches 0°C. It remains at 0°C until all the ice melts into water.

• After all the ice melts, if more heat is applied, the water’s temperature increases until it reaches 100°C, where it begins to vaporize.

• The graph of temperature vs. time shows the melting point line and boiling point line where the temperature remains constant until the phase change is complete.

Key Points:

• Pure substances have a specific melting point and boiling point.

• Impure substances have a lower melting point and may have a boiling point range.

• Boiling and melting points can be used to determine the purity of a substance.

• Heat is involved in both melting and boiling, but the temperature does not increase until the phase change is complete.