CHEM1001 Lecture 5: Chem 5

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1 Aug 2016
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Lecture 5
When a solid is heated above its melting point, it becomes liquid because the pressure is higher
than the triple point of the substance.
Intermolecular forces are still important, but the molecules have enough energy to move
around, which makes the structure mobile.
This means that a liquid is not definite in shape but rather conforms to the shape of its
container.
Its volume is usually greater than that of its corresponding solid.
The highest temperature at which a particular liquid can exist is called its critical temperature.
A liquid can be converted to a gas through heating at constant pressure to the substance's
boiling point or through reduction of pressure at constant temperature.
This process of a liquid changing to a gas is called evaporation.
Gas molecules have either very weak bonds or no bonds at all, so they can move freely and
quickly.
Because of this, not only will a gas conform to the shape of its container, it will also expand to
completely fill the container.
Gas molecules have enough kinetic energy that the effect of intermolecular forces is small, and
they are spaced very far apart from each other; the typical distance between neighboring
molecules is much greater than the size of the molecules themselves.
A gas at a temperature below its critical temperature can also be called a vapor.
A vapor can be liquefied through compression without cooling.
It can also exist in equilibrium with a liquid, in which case the gas pressure equals the vapor
pressure of the liquid.
A supercritical fluid is a gas whose temperature and pressure are greater than the critical
temperature and critical pressure.
In this state, the distinction between liquid and gas disappears.
A supercritical fluid has the physical properties of a gas, but its high density lends it the
properties of a solvent in some cases.
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