Statement A: The temperature of the liquid becomes constant once it starts boiling

This statement is true. When a liquid reaches its boiling point, it starts to vaporize and change into a gas. During this process, the temperature of the liquid remains constant.

- Boiling Point:
The boiling point is the temperature at which a substance changes from a liquid to a gas. At this temperature, the vapor pressure of the liquid is equal to the atmospheric pressure. Different substances have different boiling points. For example, water boils at 100 degrees Celsius (212 degrees Fahrenheit) at sea level.

- Vaporization:
When a liquid is heated, its molecules gain energy and move more rapidly. At the boiling point, the molecules have enough energy to overcome the attractive forces between them and escape into the air. This process is called vaporization.

- Constant Temperature:
During the phase change from liquid to gas, the temperature of the liquid remains constant. This is because the heat energy being supplied to the liquid is being used to break the intermolecular bonds and convert the liquid into a gas, rather than increasing the temperature. Once all of the liquid has vaporized, further heating will increase the temperature of the gas.

- Boiling Process:
When a liquid is heated, it first reaches its boiling point. At this point, bubbles of vapor form within the liquid and rise to the surface. These bubbles are composed of the gas phase of the substance. The temperature of the liquid remains constant until all of it has been converted into gas.

Statement B: Pressure of air at sea level is 70cm

This statement is false. The pressure of air at sea level is not measured in centimeters. The standard unit of pressure is the pascal (Pa) or other units such as atmospheres (atm) or millimeters of mercury (mmHg).

- Pressure at Sea Level:
At sea level, the average atmospheric pressure is approximately 101.3 kilopascals (kPa) or 1 atmosphere (atm). This corresponds to the weight of the column of air above a given area. The pressure decreases with increasing altitude.

- Units of Pressure:
The most commonly used unit of pressure is the pascal (Pa), which is equal to one newton per square meter. Other units such as atmospheres (atm) and millimeters of mercury (mmHg) are also used. However, centimeters (cm) is not a standard unit for measuring pressure.

In conclusion, Statement A is true because the temperature of a liquid does indeed become constant once it starts boiling. However, Statement B is false because the pressure of air at sea level is not 70 cm; it is approximately 101.3 kPa or 1 atm.