Lesson 2 - Gas Laws
This lesson considers gases and their behaviour, both on their own and as mixtures with other gases. It explores a series of gas laws which will then be used to describe the behaviour.
Dalton’s Law of Gas Mixtures
Mixtures of non-reacting gases literally surround us. Air, after all, is a mixture of predominantly nitrogen, oxygen, carbon dioxide, water vapour, and trace amounts of other gases. Beer tanks (fermenters, maturation vessels, and bright beer tanks) and containers (bottles, cans, and kegs) often contain quantities of air and carbon dioxide before being filled with beer. The pressure of the gas and the composition of the gas space above the beer has a significant influence on the dissolution of gases into and out of the beer.
Let us have a look at how these gas mixtures behave.
Non-reacting gas mixtures
The first thing to be aware of is that we are specifically dealing with mixtures of gases which do not react with one another. So, take air as an example: the oxygen, nitrogen, carbon dioxide (and other gases) present in air do not chemically react with one another at the pressures and temperatures which we utilise in a brewery.
An example of a reacting gas mixture would be the burning of a gaseous fuel, such as methane, in air. This would be a particular type of gas behaviour which is not relevant to us currently.
Before we launch into another gas law, consider the following situation which we have seen before.
Take a gas-tight container into which we introduce a quantity of pure carbon dioxide so that only carbon dioxide is present (i.e. we flushed any other gas out of the container before filling with pure CO2). As we introduce greater quantities of CO2 into the container, we know from our previous consideration of gas laws that the gas pressure will increase. We stop filling at a pressure of 2 bar a, and we disconnect the CO2 supply and connect a pure nitrogen (N2) gas supply. Consider these questions:
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What would the minimum nitrogen pressure have to be to start introducing nitrogen into the container?
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Once nitrogen is introduced into the container, and the container pressure reaches 2.1 bar a. What can you say about the relative quantities of CO2 and N2?
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If filling with N2 continued until the container pressure reached 10 bar a, would you revise your comment (in 2 above) about the relative quantities of the gases?
Give yourself 5-10 mins to think about this and then click ‘Next’ to continue with this lesson.