Phase Diagram question

[integralx2]

---

Members don't see this ad.

I dont want to pointlessly memorize this. But I am trying to understand this. Why is it at high pressure a substance is a solid ? What governs this, i know that P=F/A (P=pressure, F=force, A=area), and since its a solid the force F is stronger so it has to have an increase in pressure since there proportional ? Or does it have something to do with INTERmolecular forces ? Also same question for solid to a gas.

 

[SN2ed]

Thread moved. Threads like this belong in the MCAT Study Q&A forum.

 

[shffl]

The force exerted on a substance by pressure compacts the molecules together. The greater the pressure is, the more force and therefore the molecules are so compact it became more solid.

 

[nuD]

Here's how I think of it:

Any substance (solid, liquid, or gas) has molecules moving around. In a gas, these molecules are more or less free to move about however they want. In a liquid, intermolecular forces hold these molecules relatively close together, but the liquid can still flow freely. In a solid, the molecules have very little room to move around and they essentially vibrate in a small space.

So if you take a gas which has its molecules spread out and moving freely, and compress it with pressure, eventually the molecules will be compressed down into such a small area that they will be unable to move much at all. The result will be a solid.

 

[TwoPaddles]

^ which is also a nonspontaneous process (-ΔS) because you would want more freedom; you would want the molecules to be spread out more freely, which is not the case in a solid.

 

[Rabolisk]

^ which is also a nonspontaneous process (-ΔS) because you would want more freedom; you would want the molecules to be spread out more freely, which is not the case in a solid.

Well, yes and no. Condensation and desposition are spontaneous at low temperature, and actually increase the entropy of the universe. It's true that ΔS is always negative, but that doesn't define spontaneity. ΔS only measures how the entropy of the system changes.

 

[NYR56]

Well, yes and no. Condensation and desposition are spontaneous at low temperature, and actually increase the entropy of the universe. It's true that ΔS is always negative, but that doesn't define spontaneity. ΔS only measures how the entropy of the system changes.

This. Spontaneity is defined by Gibbs free energy, not by entropy. That's a pretty important fact to know for the MCAT. At low temperatures, the enthalpy component of G is more significant than the entropy, whereas at high temperatures, the entropy component is more significant. This relationship is specifically what drives phase changes.