# What are the theories explaining dalton's law of partial pressures?

Why does every gas behave as if it were alone in the container/ the molecules of each gas do not affect eachother? And I read in my textbook that Dalton's law is accurate with low pressures and it seems to break under high pressures. What's the reason for that?

• The simplest reason for deviations at high pressure is that the molecules have a volume of their own. See the reasons for the Van der Waals equation of state.
– user137289
Dec 28, 2016 at 8:34

Why does every gas behave as if it were alone in the container/ the molecules of each gas do not affect each other ?

A partial pressure is the effective contribution to total pressure that the gas makes to the total, allowing for the numerical balance of each.

A partial pressure calculation is equivalent to a container of the same size filled with one gas, but increasing the number of moles of that gas to match the number of moles of all gases that were in the original container.

I think people sometimes forget to allow for that numerical rebalancing when thinking about the calculation.

The calculation works because we are treating them as Ideal gases, which do not interact with each other. In this model, the type of gas does not matter apart from it's molecular mass. In more complex models we would have to allow for relative size of the particles and even their interactions ( forces between them ).

And I read in my textbook that Dalton's law is accurate with low pressures and it seems to break under high pressures. What's the reason for that ?

Pieter already answered this his comment. I'd only add that real gases have interactions that means they no longer behave like an ideal gas. The closer ( higher density, higher pressure ) they are the greater the effect of these interactions as the interactions are more more intense at closer ranges. The ideal gas laws make no allowance for these effects.

• Thank you. What are these interactions between the molecules? Dec 28, 2016 at 12:14
• Mostly this is covered by Van der Waals force. Sometimes "gas" laws are applied to materials that do no seem like gases ( liquids, plasma, planetary cores ) and in these situations other effects ( even quantum effects ) can be required. Dec 28, 2016 at 12:49