Chemistry » Chemical Equilibrium » What Is Chemical Equilibrium?

Open and Closed Systems

Open and closed systems

The important thing for the students to understand in this experiment is that there is constant movement. Molecules at the surface of the water in the closed beaker are constantly evaporating, and gas molecules are constantly condensing. There is no change in the level of the liquid and the total number molecules in the liquid and the gas state remain constant.

Point out that the level of water in the sealed container also drops initially, before finally remaining constant. This concept will help learners understand how the rate of the forward reaction decreases until equilibrium is reached.

Optional Experiment: Liquid-gas phase equilibrium


  • 2 beakers, glass cover

  • water


  1. Half-fill two beakers with water and mark the level of the water in each case.

  2. Cover one of the beakers with a glass cover.

  3. Leave the beakers and, over the course of a day or two, observe how the water levels in the two beakers change. What do you notice? Note: You could speed up this demonstration by placing the two beakers over a Bunsen burner, or in direct sunlight, to heat the water.


You should notice that in the beaker that is uncovered, the water level drops more quickly than in the covered beaker. This is because of evaporation. In the beaker that is covered, there is an initial drop in the water level, but after a while evaporation appears to stop and the water level in this beaker is higher than that in the one that is open.



In the first beaker, liquid water becomes water vapour as a result of evaporation and the water level drops. A small amount of gas molecules will condense again, but because the gas molecules can escape from the system there is much less condensation than evaporation.

In the second beaker, evaporation also takes place. However, in this case, the vapour comes into contact with the surface of the glass cover and it cools and condenses to form liquid water again. This water is returned to the beaker. Once condensation has begun, the rate at which the water level drops will start to decrease. At some point, the rate of evaporation will be equal to the rate of condensation, and there will be no change in the water level in the beaker. This can be represented as follows:

\(\text{liquid} \rightleftharpoons \text{vapour}\)

In this example, the reaction (in this case, a change in the phase of water) can proceed in either direction. In the forward direction there is a change in phase from liquid to gas, present here as water vapour. A reverse change can also take place, when vapour condenses to form liquid again.


Evaporation is when a substance goes from the liquid phase to the gas phase (it evaporates). Condensation is when a substance goes from the gas phase to the liquid phase (it condenses).

The bottle contains water vapour (gas) and water droplets are condensing on the side of the bottle.

An open system is one in which matter or energy can flow into or out of the system. In the liquid-gas demonstration we used, the first beaker was an example of an open system because the beaker could be heated or cooled (a change in energy), and water vapour (the matter) could evaporate from the beaker.

Definition: Open system

An open system is one whose borders allow the movement of energy and matter into and out of the system.

A closed system is one in which energy can enter or leave, but matter cannot. The second beaker with the glass cover is an example of a closed system. The beaker can still be heated or cooled, but water vapour cannot leave the system because the glass cover is a barrier. Condensation changes the vapour to liquid and returns it to the beaker. In other words, there is no loss of matter from the system.


Generally, some of the molecules at the surface of liquids and solids are moving into the gas phase. This means that matter is leaving the system. However, this is such a small fraction of the total volume of the liquid or solid that a reaction involving only solids or liquids can be considered a closed system.


It is useful to simplify situations in science by dividing the world into the system we are studying, and the surrounding environment that might influence the reaction, but is not part of it.

Definition: Closed system

A closed system is one in which only energy can move into and out of the system. Matter cannot be gained by the system nor lost from the system.

In a closed system it is possible for reactions to be reversible, such as in the demonstration above. In a closed system, it is also possible for a chemical reaction to reach equilibrium.

[Attributions and Licenses]

This is a lesson from the tutorial, Chemical Equilibrium and you are encouraged to log in or register, so that you can track your progress.

Log In

Share Thoughts