The States of Matter

Solid : Liquid : Gas : Plasma

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Introduction

Matter is made up of atoms or molecules. The arrangement of these particles determines the state of matter. There are four recognised states of matter: solid, liquid, gas and plasma. Matter can change between states when the temperature or pressure is changed. The chemical properties of materials are unchanged when the state changes. State changes of matter are physical rather than chemical changes.

This essay will talk of atoms but everything could be applied to molecules.

The Solid State

Solids have a fixed volume and shape.

In a solid, the atoms (or molecules) are in fixed positions relative to one another. They vibrate but stay in relative position. When the solid is heated the atoms vibrate faster. This causes the solid to grow slightly in size. It is said to expand. All solids expand on heating, especially metals. Railway lines can buckle on hot days because of this expansion.

If the atoms are arranged in a regular sequence, a crystal results. Metals and salts are crystaline. If the atoms are arranged haphazardly the solid is said to be amorphous (Greek for without shape). Glass is a good example of an amorphous solid.

Metals are crystaline because their atoms are arranged in a regular sequence. However, atoms of metals tend to have loose outer electrons. These electrons can move between atoms. This causes the metal to conduct electricity.

The Liquid State

Liquids have a fixed volume but take the shape of the container.

In a liquid, the atoms move relative to one another but still stay close together.

Liquids can only exist for a limited temperature range. For example water is a liquid for 100 degrees (Celsius). For most temperatures it is either a solid or a gas. At low pressures, the range of temperature for a liquid to exist is smaller. That is why water boils at less than 100OC at high altitudes. If the pressure is low enough, the liquid phase does not happen. This is further examined by using phase diagrams.

A pure liquid contains only one substance. Most liquids dissolve other substances. The liquid is the solvent and the substance being dissolved is the solute. The resulting mixture is called a solution.

Water is an excellent solvent. Solids (like common salt), other liquids (like alcohol) and gases (like carbon dioxide) can all dissolve in water. They are said to be soluble in water.

Many substances do not dissolve in water. They are said to be insoluble. Oils are usually insoluble in water. When a mixture of oil and water is shaken, the result is an emulsion. If an insoluble solid is mixed with water, the result is called a suspension.

Pure water does not conduct electricity. Many solutions do conduct electricity. Adding salt (sodium chloride) to pure water makes the latter conduct electricity. This is because the salt breaks up into ions. These ions can move and this causes the solution to conduct electricity.

Passing an electric current through a conducting liquid or solution causes the ions to move. This breaks up compounds into their constituent elements, a process called electrolysis.

The Gas State

Gases have no fixed volume or shape; they expand to fill all available space.

Atoms in a gas are free to move independently from each other. Because the atoms in a gas are moving at random, the gaseous state is the simplest to describe mathematically. There are three gas laws.

If the volume of a gas is held constant,
its pressure is proportional to the temperature.

Pressure proportional to Temperature

As the temperature increases, the atoms move faster and strike the walls of the container with more force. This manifests itself as an increase in pressure.

If the pressure of a gas is held constant,
its volume is proportional to the temperature.

Volume proportional to Temperature

As the temperature increases, the atoms move faster and require more space so that they strike the walls of the container with the same net force. The volume increases.

If the temperature of a gas is held constant,
its pressure is inversely proportional to the volume.

Pressure inversley proportional to Volume

If the volume of a gas is decreased, then the same number of atoms moving at the same speed (because the temperature has not changed) will strike the walls of the container more often. This manifests itself as an increase in pressure.

Phase Diagrams

Phase diagrams look at how a substance changes state as pressure and temperature vary. An example is shown below.

Phase Diagram

In this diagram, pressure increases upwards, temperature increases to the right. The triple point is the temperature where the three states (solid, liquid, gas) exist together.

If the pressure is above the triple point the substance behaves as follows: For a given pressure, a substance is a solid at low temperature. As its temperature rises it reaches a point where it turns to a liquid. This is called the melting point. If the liquid's temperature rises further it eventually reaches a temperature where it turns into a gas. This is the boiling point. If the pressure is below the triple point the following happens: The substance is a solid at lower temperatures. When heated the substance changes directly to a gas. This process is called sublimation. This temperature is called the sublimation point.

The melting, boiling and sublimation points of a substance depend on the pressure.

Below is the phase diagram for water.

Phase Diagram for Water

At atmospheric pressure, water is above its triple point. This means that water has a melting point (0OC) and a boiling point (100OC). At lower pressure, the melting point is slightly higher and the boiling point is a lot less.

In contrast, the phase diagram for carbon dioxide is shown below.

Phase Diagram for Carbon Dioxide

At atmospheric pressure, carbon dioxide is below its triple point. This means that carbon dioxide sublimates. Its sublimation point is -80OC. This is why it is called dry ice.

Plasma

Plasma is a gas that is so hot that it has ionised. The gas is electrically charged and is affected by magnetic and electric fields. The sun's corona and the particles that cause the aurora are both made from plasma.

© 2001, 2009 KryssTal


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