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States of matter and the kinetic molecular theory

Module by: Free High School Science Texts Project. E-mail the author

Introduction

In this chapter we will explore the states of matter and then look at the kinetic molecular theory. Matter exists in three states: solid, liquid and gas. We will also examine how the kinetic theory of matter helps explain boiling and melting points as well as other properties of matter.

Note:

When a gas is heated above a certain temperature the electrons in the atoms start to leave the atoms. The gas is said to be ionised. When a gas is ionised it is known as a plasma. Plasmas share many of the properties of gases (they have no fixed volume and fill the space they are in). This is a very high energy state and plasmas often glow. Ionisation is the process of moving from a gas to a plasma and deionisation is the reverse process. We will not consider plasmas further in this chapter.

States of matter

All matter is made up of particles. We can see this when we look at diffusion. Diffusion is the movement of particles from a high concentration to a low concentration. Diffusion can be seen as a spreading out of particles resulting in an even distribution of the particles. You can see diffusion when you place a drop of food colouring in water. The colour slowly spreads out through the water. If matter were not made of particles then we would only see a clump of colour when we put the food colouring in water, as there would be nothing that could move about and mix in with the water. The composition of matter will be looked at in What are the objects around us made of?.

Diffusion is a result of the constant thermal motion of particles. In Section 4 we will talk more about the thermal motion of particles.

In 1828 Robert Brown observed that pollen grains suspended in water moved about in a rapid, irregular motion. This motion has since become known as Brownian motion. Brownian motion is essentially diffusion of many particles.

Matter exists in one of three states, namely solid, liquid and gas. Matter can change between these states by either adding heat or removing heat. This is known as a change of state. As we heat an object (e.g. water) it goes from a solid to a liquid to a gas. As we cool an object it goes from a gas to a liquid to a solid. The changes of state that you should know are:

  • Melting: Melting is the process of going from solid to liquid.
  • Boiling: Boiling (or evaporation) is the process of going from liquid to gas.
  • Freezing: Freezing is the process of going from liquid to solid.
  • Condensation: Condensation is the process of going from gas to liquid.
  • Sublimation: Occasionally (e.g. for carbon dioxide) we can go directly from solid to gas in a process called sublimation.
A solid has a fixed shape and volume. A liquid takes on the shape of the container that it is in. A gas completely fills the container that it is in. See Section 4 for more on changes of state.

If we know the melting and boiling point of a substance then we can say what state (solid, liquid or gas) it will be in at any temperature.

Experiment: States of matter

Aim

To investigate the heating and cooling curve of water.

Apparatus

beakers, ice, bunsen burner, thermometer, water.

Method

  • Place some ice in a beaker
  • Measure the temperature of the ice and record it.
  • After 10 s measure the temperature again and record it. Repeat every 10 s, until at least 1 minute after the ice has melted.
  • Heat some water in a beaker until it boils. Measure and record the temperature of the water.
  • Remove the water from the heat and measure the temperature every 10 s, until the beaker is cool to touch

Warning:

Be careful when handling the beaker of hot water. Do not touch the beaker with your hands, you will burn yourself.

Results

Record your results in the following table:

Table 1: Table of results
Temperature of ice Time (s) Temperature of water Time (s)
       
       
       
       
       
Plot a graph of temperature against time for the ice melting and the boiling water cooling.

Discussion and conclusion

Discuss your results with others in your class. What conclusions can you draw? You should find that the temperature of the ice increases until the first drops of liquid appear and then the temperature remains the same, until all the ice is melted. You should also find that when you cool water down from boiling, the temperature remains constant for a while, then starts decreasing.

In the above experiment, you investigated the heating and cooling curves of water. We can draw heating and cooling curves for any substance. A heating curve of a substance gives the changes in temperature as we move from a solid to a liquid to a gas. A cooling curve gives the changes in temperature as we move from gas to liquid to solid. An important observation is that as a substance melts or boils, the temperature remains constant until the substance has changed state. This is because all the heat energy goes into breaking or forming the forces between the molecules.

The above experiment is one way of demonstrating the changes of state of a substance. Ice melting or water boiling should be very familiar to you.

The Kinetic Theory of Matter

The kinetic theory of matter helps us to explain why matter exists in different phases (i.e. solid, liquid and gas), and how matter can change from one phase to the next. The kinetic theory of matter also helps us to understand other properties of matter. It is important to realise that what we will go on to describe is only a theory. It cannot be proved beyond doubt, but the fact that it helps us to explain our observations of changes in phase, and other properties of matter, suggests that it probably is more than just a theory.

Broadly, the Kinetic Theory of Matter says that:

  • Matter is made up of particles that are constantly moving.
  • All particles have energy, but the energy varies depending on whether the substance is a solid, liquid or gas. Solid particles have the least amount of energy and gas particles have the greatest amount of energy.
  • The temperature of a substance is a measure of the average kinetic energy of the particles.
  • A change in phase may occur when the energy of the particles is changed.
  • There are spaces between the particles of matter.
  • There are attractive forces between particles and these become stronger as the particles move closer together. These attractive forces will either be intramolecular forces (if the particles are atoms) or intermolecular forces (if the particles are molecules). When the particles are extremely close, repulsive forces start to act.

Table 2 summarises the characteristics of the particles that are in each phase of matter.

Table 2: Table summarising the general features of solids, liquids and gases.
Property of matter Solid Liquid Gas
Particles Atoms or molecules Atoms or molecules Atoms or molecules
Energy and movement of particles Low energy - particles vibrate around a fixed point Particles have less energy than in the gas phase Particles have high energy and are constantly moving
Spaces between particles Very little space between particles. Particles are tightly packed together Smaller spaces than in gases, but larger spaces than in solids Large spaces because of high energy
Attractive forces between particles Very strong forces. Solids have a fixed volume. Stronger forces than in gas. Liquids can be poured. Weak forces because of the large distance between particles
Changes in phase Solids become liquids if their temperature is increased. In some cases a solid may become a gas if the temperature is increased. A liquid becomes a gas if its temperature is increased. It becomes a solid if its temperature decreases. In general a gas becomes a liquid when it is cooled. (In a few cases a gas becomes a solid when cooled). Particles have less energy and therefore move closer together so that the attractive forces become stronger, and the gas becomes a liquid (or a solid.)

The following presentation is a brief summary of the above. Try to fill in the blank spaces before clicking onto the next slide.

Figure 1

Let's look at an example that involves the three phases of water: ice (solid), water (liquid) and water vapour (gas). Note that in the Figure 2 below the molecules in the solid phase are represented by single spheres, but they would in reality look like the molecules in the liquid and gas phase. Sometimes we represent molecules as single spheres in the solid phase to emphasise the small amount of space between them and to make the drawing simpler.

Figure 2: The three phases of matter
Figure 2 (CG10C2_008.png)

Taking water as an example we find that in the solid phase the water molecules have very little energy and can't move away from each other. The molecules are held closely together in a regular pattern called a lattice. If the ice is heated, the energy of the molecules increases. This means that some of the water molecules are able to overcome the intermolecular forces that are holding them together, and the molecules move further apart to form liquid water. This is why liquid water is able to flow, because the molecules are more free to move than they were in the solid lattice. If the molecules are heated further, the liquid water will become water vapour, which is a gas. Gas particles have lots of energy and are far away from each other. That is why it is difficult to keep a gas in a specific area! The attractive forces between the particles are very weak and they are only loosely held together. Figure 3 shows the changes in phase that may occur in matter, and the names that describe these processes.

Figure 3: Changes in phase
Figure 3 (CG10C2_009.png)

Intramolecular and intermolecular forces (Not in CAPS - Included for Completeness)

When atoms join to form molecules, they are held together by chemical bonds. The type of bond, and the strength of the bond, depends on the atoms that are involved. These bonds are called intramolecular forces because they are bonding forces inside a molecule ('intra' means 'within' or 'inside'). Sometimes we simply call these intramolecular forces chemical bonds.

Definition 1: Intramolecular force
The force between the atoms of a molecule, which holds them together.

Examples of the types of chemical bonds that can exist between atoms inside a molecule are shown below. These will be looked at in more detail in Chemical bonding.

  • Covalent bond Covalent bonds exist between non-metal atoms e.g. There are covalent bonds between the carbon and oxygen atoms in a molecule of carbon dioxide.
  • Ionic bond Ionic bonds occur between non-metal and metal atoms e.g. There are ionic bonds between the sodium and chlorine atoms in a molecule of sodium chloride.
  • Metallic bond Metallic bonds join metal atoms e.g. There are metallic bonds between copper atoms in a piece of copper metal.

Intermolecular forces are those bonds that hold molecules together. A glass of water for example, contains many molecules of water. These molecules are held together by intermolecular forces. The strength of the intermolecular forces is important because they affect properties such as melting point and boiling point. For example, the stronger the intermolecular forces, the higher the melting point and boiling point for that substance. The strength of the intermolecular forces increases as the size of the molecule increases.

Definition 2: Intermolecular force
A force between molecules, which holds them together.

The following diagram may help you to understand the difference between intramolecular forces and intermolecular forces.

Figure 4: Two representations showing the intermolecular and intramolecular forces in water: space-filling model and structural formula.
Figure 4 (CG10C2_007.png)

It should be clearer now that there are two types of forces that hold matter together. In the case of water, there are intramolecular forces that hold the two hydrogen atoms to the oxygen atom in each molecule of water (these are the solid lines in the above diagram). There are also intermolecular forces between each of these water molecules. These intermolecular forces join the hydrogen atom from one molecule to the oxygen atom of another molecule (these are the dashed lines in the above figure). As mentioned earlier, these forces are very important because they affect many of the properties of matter such as boiling point, melting point and a number of other properties. Before we go on to look at some of these examples, it is important that we first take a look at the Kinetic Theory of Matter.

Tip:

To help you remember that intermolecular means between molecules, remember that international means between nations.

Intramolecular and intermolecular forces

  1. Using ammonia gas as an example...
    1. Explain what is meant by an intramolecular force or chemical bond.
    2. Explain what is meant by an intermolecular force.
    Click here for the solution
  2. Draw a diagram showing three molecules of carbon dioxide. On the diagram, show where the intramolecular and intermolecular forces are.
    Click here for the solution
  3. Why is it important to understand the types of forces that exist between atoms and between molecules? Try to use some practical examples in your answer.
    Click here for the solution

The Properties of Matter

Let us now look at what we have learned about chemical bonds, intermolecular forces and the kinetic theory of matter, and see whether this can help us to understand some of the macroscopic properties of materials.

  1. Melting point
    Definition 3: Melting point
    The temperature at which a solid changes its phase or state to become a liquid. The process is called melting and the reverse process (change in phase from liquid to solid) is called freezing.
    In order for a solid to melt, the energy of the particles must increase enough to overcome the bonds that are holding the particles together. It makes sense then that a solid which is held together by strong bonds will have a higher melting point than one where the bonds are weak, because more energy (heat) is needed to break the bonds. In the examples we have looked at metals, ionic solids and some atomic lattices (e.g. diamond) have high melting points, whereas the melting points for molecular solids and other atomic lattices (e.g. graphite) are much lower. Generally, the intermolecular forces between molecular solids are weaker than those between ionic and metallic solids.
  2. Boiling point
    Definition 4: Boiling point
    The temperature at which a liquid changes its phase to become a gas. The process is called evaporation and the reverse process is called condensation
    When the temperature of a liquid increases, the average kinetic energy of the particles also increases and they are able to overcome the bonding forces that are holding them in the liquid. When boiling point is reached, evaporation takes place and some particles in the liquid become a gas. In other words, the energy of the particles is too great for them to be held in a liquid anymore. The stronger the bonds within a liquid, the higher the boiling point needs to be in order to break these bonds. Metallic and ionic compounds have high boiling points while the boiling point for molecular liquids is lower. The data in Table 3 below may help you to understand some of the concepts we have explained. Not all of the substances in the table are solids at room temperature, so for now, let's just focus on the boiling points for each of these substances. What do you notice?
    Table 3: The melting and boiling points for a number of substances
    SubstanceMelting point (° C° C)Boiling point ( °C °C)
    Ethanol (C2H6OC2H6O)- 114,378,4
    Water0 100
    Mercury-38,83 356,73
    Sodium chloride801 1465
    You will have seen that substances such as ethanol, with relatively weak intermolecular forces, have the lowest boiling point, while substances with stronger intermolecular forces such as sodium chloride and mercury, must be heated much more if the particles are to have enough energy to overcome the forces that are holding them together in the liquid. See the section below for a further exercise on boiling point.
  3. Density and viscosity

    Note:

    Density and viscosity is not in CAPS - Included for Completeness
    Definition 5: Density
    Density is a measure of the mass of a substance per unit volume.
    The density of a solid is generally higher than that of a liquid because the particles are held much more closely together and therefore there are more particles packed together in a particular volume. In other words, there is a greater mass of the substance in a particular volume. In general, density increases as the strength of the intermolecular forces increases.
    Definition 6: Viscosity
    Viscosity is a measure of how resistant a liquid is to flowing (in other words, how easy it is to pour the liquid from one container to another).
    Viscosity is also sometimes described as the 'thickness' of a fluid. Think for example of syrup and how slowly it pours from one container into another. Now compare this to how easy it is to pour water. The viscosity of syrup is greater than the viscosity of water. Once again, the stronger the intermolecular forces in the liquid, the greater its viscosity.

It should be clear now that we can explain a lot of the macroscopic properties of matter (i.e. the characteristics we can see or observe) by understanding their microscopic structure and the way in which the atoms and molecules that make up matter are held together.

Exercise: Forces and boiling point

The table below gives the molecular formula and the boiling point for a number of organic compounds called alkanes (more on these compounds in grade 12). Refer to the table and then answer the questions that follow.

Table 4
Organic compoundMolecular formulaBoiling point ( °C °C)
Methane CH4CH4-161.6
EthaneC2H6C2H6- 88.6
PropaneC3H8C3H8 -45
Butane C4H10C4H10-0.5
PentaneC5H12C5H12 36.1
Hexane C6H14C6H1469
HeptaneC7H16C7H16 98.42
Octane C8H18C8H18125.52
Data from: http://www.wikipedia.com
  1. Draw a graph to show the relationship between the number of carbon atoms in each alkane and its boiling point. (Number of carbon atoms will go on the x-axis and boiling point on the y-axis).
  2. Describe what you see.
  3. Suggest a reason for what you have observed.
  4. Why was it enough for us to use 'number of carbon atoms' as a measure of the molecular weight of the molecules?
Click here for the solution

Investigation : Determining the density of liquids:

Density is a very important property because it helps us to identify different materials. Every material, depending on the elements that make it up and the arrangement of its atoms, will have a different density.

The equation for density is:

Density=MassVolumeDensity=MassVolume
(1)

Discussion questions:

To calculate the density of liquids and solids, we need to be able to first determine their mass and volume. As a group, think about the following questions:

  • How would you determine the mass of a liquid?
  • How would you determine the volume of an irregular solid?

Apparatus:

Laboratory mass balance, 10ml10ml and 100ml100ml graduated cylinders, thread, distilled water, two different liquids.

Method:

Determine the density of the distilled water and two liquids as follows:

  1. Measure and record the mass of a 10ml10ml graduated cyclinder.
  2. Pour an amount of distilled water into the cylinder.
  3. Measure and record the combined mass of the water and cylinder.
  4. Record the volume of distilled water in the cylinder
  5. Empty, clean and dry the graduated cylinder.
  6. Repeat the above steps for the other two liquids you have.
  7. Complete the table below.
Table 5
Liquid Mass (g) Volume (ml) Density ( g·ml -1g·ml -1)
Distilled water      
Liquid 1      
Liquid 2      

Investigation : Determining the density of irregular solids:

Apparatus:

Use the same materials and equpiment as before (for the liquids). Also find a number of solids that have an irregular shape.

Method:

Determine the density of irregular solids as follows:

  1. Measure and record the mass of one of the irregular solids.
  2. Tie a piece of thread around the solid.
  3. Pour some water into a 100ml100ml graduated cylinder and record the volume.
  4. Gently lower the solid into the water, keeping hold of the thread. Record the combined volume of the solid and the water.
  5. Determine the volume of the solid by subtracting the combined volume from the original volume of the water only.
  6. Repeat these steps for the second object.
  7. Complete the table below.
Table 6
Solid Mass (g) Volume (ml) Density ( g·ml -1g·ml -1)
Solid 1      
Solid 2      
Solid 3      

Figure 5

Summary

  • There are three states of matter: solid, liquid and gas.
  • Diffusion is the movement of particles from a high concentration to a low concentration. Brownian motion is the diffusion of many particles.
  • The kinetic theory of matter attempts to explain the behaviour of matter in different phases.
  • The kinetic theory of matter says that all matter is composed of particles which have a certain amount of energy which allows them to move at different speeds depending on the temperature (energy). There are spaces between the particles and also attractive forces between particles when they come close together.
  • Intramolecular force is the force between the atoms of a molecule, which holds them together. Intermolecular force is a force between molecules, which holds them together.
  • Understanding chemical bonds, intermolecular forces and the kinetic theory of matter can help to explain many of the macroscopic properties of matter.
  • Melting point is the temperature at which a solid changes its phase to become a liquid. The reverse process (change in phase from liquid to solid) is called freezing. The stronger the chemical bonds and intermolecular forces in a substance, the higher the melting point will be.
  • Boiling point is the temperature at which a liquid changes phase to become a gas. The reverse process (change in phase from gas to liquid) is called condensing. The stronger the chemical bonds and intermolecular forces in a substance, the higher the boiling point will be.
  • Density is a measure of the mass of a substance per unit volume.
  • Viscosity is a measure of how resistant a liquid is to flowing.

End of chapter exercises

  1. Give one word or term for each of the following descriptions.
    1. The property that determines how easily a liquid flows.
    2. The change in phase from liquid to gas.
    Click here for the solution
  2. If one substance A has a melting point that is lower than the melting point of substance B, this suggests that...
    1. A will be a liquid at room temperature.
    2. The chemical bonds in substance A are weaker than those in substance B.
    3. The chemical bonds in substance A are stronger than those in substance B.
    4. B will be a gas at room temperature.
    Click here for the solution
  3. Boiling point is an important concept to understand.
    1. Define 'boiling point'.
    2. What change in phase takes place when a liquid reaches its boiling point?
    3. What is the boiling point of water?
    4. Use the kinetic theory of matter and your knowledge of intermolecular forces to explain why water changes phase at this temperature.
    Click here for the solution
  4. Describe a solid in terms of the kinetic molecular theory.
    Click here for the solution
  5. Refer to the table below which gives the melting and boiling points of a number of elements and then answer the questions that follow. (Data from http://www.chemicalelements.com)
    Table 7
    ElementMelting pointBoiling point (°C°C)
    copper10832567
    magnesium6501107
    oxygen-218,4-183
    carbon35004827
    helium-272-268,6
    sulphur112,8444,6
    1. What state of matter (i.e. solid, liquid or gas) will each of these elements be in at room temperature?
    2. Which of these elements has the strongest forces between its atoms? Give a reason for your answer.
    3. Which of these elements has the weakest forces between its atoms? Give a reason for your answer.
    Click here for the solution

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