Dalton's law of partial pressure John Dalton studied the effect of gases in a mixture. He observed that the Total Pressure of a gas mixture was the sum of the Partial Pressure of each gas.
The Partial Pressure is defined as the pressure of a single gas in the mixture as if that gas alone occupied the container. In other words, Dalton maintained that since there was an enormous amount of space between the gas molecules within the mixture that the gas molecules did not have any influence on the motion of other gas molecules, therefore the pressure of a gas sample would be the same whether it was the only gas in the container or if it were among other gases. This assumption that molecules act independently of one another works fine as long as there is a lot of space between gas molecules in the mixture and the temperature is not too low. Lowering the temperature and/or compressing the gas will upset that assumption. This is really the same assumption made for other Ideal Gas Laws. In fact, this assumption is a postulate of the Kinetic Molecular Theory of Gases. If the assumption breaks down then the gas does not behave as predicted by all the Ideal Gas Laws. The gas "deviates" from Ideal Gas Behavior
