Evaporation and boiling

Evaporation and boiling

What happens in a liquid during evaporation and boiling? What does its boiling point depend on?

Chemistry

Keywords

evaporation, boiling, phase transition, boiling point, pressure, heat transfer, temperature, air pressure, state of matter, water, fluid, temperature change, physical, thermodynamics, boiling water, physical property, physics, chemistry

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Scenes

Evaporation

  • unsaturated vapour - The amount of molecules leaving the liquid is greater than the amount reentering.
  • liquid
  • vapour - Evaporated liquid.
  • surface of the liquid
  • saturated vapour - The amount of molecules leaving the liquid and the amount reentering are in balance.

In liquids, the attraction between the particles is strong enough for the particles to remain in close contact as they move past one another. As they move about, they constantly collide with one another thereby transferring their energy. If some particles on the surface of the liquid receive enough energy, they may leave the liquid. This is called evaporation.

During evaporation, particles that have more energy than the average leave the liquid, so the particles that remain in the liquid have less than average energy, that is, the temperature of the liquid decreases. This is the reason why we feel cold when our body is wet. In other words, evaporation implies heat reduction.

The speed of evaporation is significantly influenced by the surface area and temperature of the liquid, the humidity of the space above the surface of the liquid, and the movement of air.

Saturated vapour

  • saturated vapour - The amount of molecules leaving the liquid and the amount reentering are in balance.
  • Saturated vapour condenses when compressed; its pressure does not increase. - The pressure of saturated vapour depends only on the temperature.
  • liquid
  • piston

When a liquid evaporates in an enclosed space, more and more evaporated particles gather above the surface of the liquid. Many of these particles, however, condense and reenter the liquid.

When the processes of evaporation and condensation reach the state of equilibrium, saturated vapour forms inside the enclosed space.

The particles of the vapour collide with the wall of the container, that is, they exert pressure on it. The vapour pressure depends only on the temperature: the higher the temperature, the higher the pressure.
When a gas is compressed, its pressure increases; however, when vapour is compressed, it condenses and its pressure remains unchanged. This is the most important difference between a gas and vapour.

Boiling

  • vapour bubbles - Vapour bubbles are formed within the boiling water.
  • liquid
  • atmospheric pressure - Its average value at sea level is about 101,000 Pa.
  • thermometer
  • pressure in the vapour bubble - The pressure in the vapour bubble depends on the temperature.
  • water molecules
  • vapour bubble

The phase transition from the liquid to the gas phase can take place in two different ways: by boiling or by evaporation.

Evaporation takes place only on the surface, while during boiling vapour bubbles form within the liquid and rise to the surface.

This process can only happen if the pressure exerted by the saturated vapour reaches the external air pressure at a given temperature, otherwise, the air pressure would make the recently formed vapour bubbles collapse.
That is, for boiling to occur, a suitably high temperature or low air pressure is needed.

Boiling point

  • Low atmospheric pressure the boiling point of water is lower
  • Normal atmospheric pressure the boiling point of water is 100 °C
  • 0 m
  • 100 °C
  • 89.6 °C
  • 3,000 m
  • 74 °C
  • 8,000 m
  • 70.6 °C
  • 10,000 m

The boiling point of liquids depends on pressure.
Water boils at 100 °C at normal atmospheric pressure. However, if the pressure is lower, for example in high mountains, the boiling point is also lower.
Under high pressure, however, the boiling point of water increases. This is why we can cook food faster, at temperatures above 100 °C, in a sealed pressure cooker.

Pressure cooker

  • high-temperature water - The temperature of a liquid in a pressure cooker is higher than the boiling point of a liquid in an open pot.
  • valve - The valve remains closed if the internal vapour pressure does not reach the required maximum pressure. If the pressure grows above the preset limit inside the pressure cooker, the valve opens, allowing steam to escape, thus keeping the pressure constant.
  • high-pressure vapour - If a liquid is heated in an enclosed space, the vapour pressure inside will be higher than the atmospheric pressure outside.
  • thermometer
  • vapour pressure

A pressure cooker is a vessel in which, when sealed, water boils at a temperature higher than its normal boiling point, that is, above 100 °C, which allows food to be cooked quicker.

The reason for the high boiling point is that the lid makes the pressure cooker airtight so the vapour cannot escape. The pressure of the vapour is much higher than the external air pressure, and because of this, the boiling point of the liquid in the vessel increases.

In one type of pressure cookers, the internal pressure is controlled by a weighted valve on top of the lid. Higher the weight of the valve, the higher the vapour pressure in the cooker, and therefore, the higher the boiling point of the liquid inside.

Cavitation

  • front of the propeller blades - high pressure
  • back of the propeller blades - low pressure - At the points of the propeller where the pressure of the water is low, the water may start to boil.

Cavitation occurs when an object, for example, a propeller, moves fast in water.
At certain points on the surface of the rapidly moving object, the liquid's pressure may decrease; as a consequence, the liquid may boil and vapour bubbles form.

If these bubbles or cavities reach a region where the pressure is higher, they collapse almost instantly. As the bubbles collapse, a significant amount of energy is released in the form of an acoustic shock wave, which, besides the loud noise, can cause damage on the surface of the object as well.
In the case of pumps and propellers, it is very important to minimise the harmful effects of cavitation. However, this phenomenon can also be utilised to clean the surface of various objects.

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