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In our previous post, we talked about the properties of gases and this article will focus on the behaviour of liquids and solids. A substance is liquid if it melts below room temperature and boils above room temperature; a substance is solid if it melts above room temperature.

General Properties Of Liquids

The following physical properties of a liquid illustrate its immediate nature. The Kinetic theory can explain each property outlined below. Below are some of those properties:

  • A liquid, like a solid, but unlike a gas, maintains its volume.
  • A liquid has no finite shape rather, it takes the shape of the container
  • Liquids are incompressible
  • Liquid particles are mobile but slow also, they can only rotate and vibrate.
  • Liquid particles diffuse but slowly
  • Liquid molecules undergo evaporation

Evaporation Of Liquids

Evaporation is a natural phenomenon that occurs in a liquid when the particles at the surface change into vapour. This phenomenon can be demonstrated by observing the level of a liquid, such as methylated spirits, placed in a beaker at room temperature. The liquid level continues to fall steadily at this temperature until the beaker becomes empty.

Boiling Points Of Liquids

The variation of vapour pressures of a liquid with temperatures shows that a rise in temperature leads to an increase in the vapour pressure, first slowly, then steeply. When a liquid is heated in a container, there is a gradual increase in the average kinetic energy of the particles; with time, the liquid starts to evaporate. When the vapour pressure of the liquid is the same as the atmospheric pressure, the liquid boils.

The normal boiling point of a liquid is the temperature at which the vapour pressure of a liquid is equal to the standard atmospheric pressure of one atmosphere, 1.01 * 105Nm-2. The moral boiling point of a liquid is a measure of the strength of the cohesive forces between the particles. The higher the normal boiling point, the stronger the cohesive forces between the particles. For instance: the normal boiling point of water is 100oC, that of ethanol is 78oC, while that of ethoxyethane(ether) is 35oC.

Boiling Points at Non-Standard Pressures

A liquid can boil at any temperature, by altering the external pressure. At a pressure lower than the atmospheric pressure, a liquid boils at a lower temperature and vice versa.

Critical Temperature & Pressure Of A Liquid

The critical temperature of the liquid is the highest temperature at which it can exist as a liquid, or at which a gas can be liquefied. The pressure required to liquefy a gas at the critical temperature is called critical pressure, while the volume occupied by one mole of a gas at critical temperature and pressure is the critical volume.

General Properties Of Solids

At the top of the page, we gave some of the general properties of liquids. The following are the properties of solids; they can be explained by the kinetic theory also:

  • Solids have a definite mass
  • Solids are practically incompressible
  • Solids have definite shape
  • Solid particles can only vibrate about their fixed positions
  • Solid particles can diffuse but rather slowly due to the weak vibratory motions of the particles.

Types Of Solids

There are generally two types of solids namely crystalline and non-crystalline (amorphous). Crystalline solids are made up of particles that are orderly arranged to give a regular geometry. Non-crystalline solids are made up of particles that are not orderly arranged and hence shapeless and with non-definite melting points.

In crystals, there are five attractive forces namely: ionic, covalent, metallic, molecular, and van der waals forces and you read about them here in this article.

Comparing Forces Of Attraction In Solids

Physical properties such as volatility, melting and boiling points of a solid depend on the strength of the forces of attraction between the atoms, molecules, or ions that make up the solids. In an ionic solid like NaCl, the forces of attraction between the opposite atoms are so strong that a large amount of heat must be absorbed before the ions can vibrate vigorously, overcome the inter-ionic forces, and form a liquid.

In molten ionic liquids, the strong inter-ionic forces still persist between the opposite ions hence, ionic solids are hard, non-volatile, have high melting and boiling points, and have high molar heat of fusion. They are good conductors of heat and electricity in a melting state. The large difference between the melting point and boiling point is due to the stronger cohesive forces between the liquid particles than between the solid particles.

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