Chemistry help?

Question

The density of a substance changes as pressure or temperature changes. Explain. Is it the mass or the volume (or both) that is changing?

Answer 1

When the pressure changes, the density increases. This is what happens when you take a sponge and squeeze it into a ball. You exert pressure on it so it shrinks. When that pressure is removed, the sponge expands to its normal volume. When the temperature increases, the substance expands and the density decreases. I don't know of a good way to explain this without getting into complicated thermodynamics. The mass of the substance never changes during these processes

Answer 2

Density is about how many molecules you have in a given volume. Have you seen the commercials about space saver bags? the one that you can use a vacuum to take the air out? The commercial shows a cluttered closet at first and then shows this lady who puts every thing in space saver bags and vacuum the air out. Now the closet doesn't look cluttered anymore and there is more room for more stuff. The more things you can fit in the closet, the "denser" the closet becomes.
The other analogy is when you are packing a luggage. You can only bag a few items in a luggage, or fill it up to its top, or try to fill it a little above its edge and try to sit on it to close the top. The more items you can put in a luggage, the denser it becomes.
It is the same thing with substances. Lets say that you have a one litter container and you have filled it with, lets say 2 moles of oxygen gas. The temperature is 20ºc and the pressure is 1atm. Now first you keep the temperature constant and change the pressure. To change the pressure you have to change the volume. If you increase the volume the pressure goes down and is you decrease the volume, the pressure goes up. The lower the pressure, the more space is in the container therefore oxygen molecules can move easier, therefore they become less dense. When the pressure increases however, there is less room in the container therefore the gas becomes more dense.
Same thing happens if you keep volume and pressure constant and change the temperature. the higher the temperature, the faster the molecules move and the further they want to get from each other. Since then there will be more space between molecules, they become less dense. And if you cool down the container, the molecules become closer to each other and take less space, therefore they become more dense.

Answer 3

Mass doesn't change at all. If you lower the pressure, volume incereases and density (which is mass divided by volume) decreases. So only density is the one that changes, mass doesn't.
If you increase the temperature, volume usually increases also. And the density decreases again.

Answer 4

Mass is the amount of matter that you have. It can not change. The volume changes to make the density change. As things get warmer, the particles spread out. less dense, more volume. Pressure tries to squeeze it together, less volume, less density.

Answer 5

It is the volume that is changing. As the pressure increases your are smashing the molecules closser so they occupy less volume. As you increase the temp the molecules are moving faster subsequently moving further apart and increasing the volume

Answer 6

PV = nRT it is the volume that is changing.
mass is constant in this equation - it doesnt change