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

When a liquid starts boiling, it means that it has reached its boiling point. The boiling point is the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure. At this point, the liquid begins to vaporize and convert into gas.

Explanation:
- During the process of boiling, heat energy is continuously added to the liquid. This heat energy increases the kinetic energy of the liquid particles, causing them to move faster and break free from the liquid's surface.
- As the liquid particles escape into the air as gas molecules, they create bubbles within the liquid. These bubbles rise to the surface and burst, releasing the gas into the atmosphere.
- The temperature of the liquid remains constant during boiling because the heat energy being added is being used to convert the liquid into gas rather than increasing the temperature.
- This constant temperature is known as the boiling point of the liquid. The boiling point varies for different liquids and depends on the atmospheric pressure.
- Once the entire liquid has converted into gas and there is no further liquid left, the temperature can start increasing again if more heat energy is added.

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

The pressure of air at sea level is not 70 cm. The correct statement is that the pressure of air at sea level is approximately 760 mmHg or 101.3 kPa.

Explanation:
- Atmospheric pressure is the force exerted by the weight of air molecules in the Earth's atmosphere.
- The pressure of air decreases with increasing altitude. At sea level, where the Earth's surface is closest to the Earth's center of gravity, the atmospheric pressure is highest.
- The standard unit of measuring atmospheric pressure is millimeters of Mercury (mmHg) or kilopascals (kPa).
- At sea level, the average atmospheric pressure is approximately 760 mmHg or 101.3 kPa.
- This pressure is equivalent to the pressure exerted by a column of mercury 760 mm high in a barometer, when balanced with the atmospheric pressure.
- As altitude increases, the atmospheric pressure decreases due to the decreasing density of air molecules. This is why, for example, people climbing mountains often notice a decrease in atmospheric pressure.

Therefore, the correct statement is B) Statement B: Pressure of air at sea level is 70 cm. is false. The pressure of air at sea level is approximately 760 mmHg or 101.3 kPa.