Imagine you have 100 people in a room that is 100 ft by 100 ft square (or 10,000 ft2
). There are 40 women and 60 men. If all these people are milling about, it would be accurate to say that the total area on average occupied by the women is 10,000 ft2
, because they can be anywhere in the room at any moment. However each person has their own little area element at any given time, and assuming that people are, on average, the same distance apart, at any given time the fraction of the total volume occupied by any given person is the total area available divided by the total number of people, or 100 ft2
. Now the total area of women (compared to that occupied by men) at this instant in time (or any instant in time) is the total available area multiplied by the fraction of people who are women - or 10,000 ft2
* 4/10, or 4,000 ft2
. The partial area of men would be then 6,000 ft2
. The partial volumes are additive because if you add up the area occupied by men plus the area occupied by women, at any instant in time, you get the total area available.
This is what is meant by partial volumes being additive, except with gas molecules you deal with volumes instead of areas. Of course gasses don't always behave ideally. In our analogy, if there are enough people in the room, maybe some of them know each other, and they stick together and have conversations. Now we can't say that every person on average is the same distance apart. Just so with real gasses. They stick together when the concentration (pressure) gets high enough, and so partial pressures and volumes are no longer additive. But we usually assume that gasses behave ideally . or that people are antisocial.