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The properties of matter

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

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 1: 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 2: 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 1 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 1: The melting and boiling points for a number of substances
    SubstanceMelting point (0 C0 C)Boiling point ( 0C 0C)
    Ethanol (C2H6O)- 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.

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 2
Organic compoundMolecular formulaBoiling point ( 0 0C)
Methane CH4-161.6
EthaneC2H6- 88.6
PropaneC3H8 -45
Butane C4H10-0.5
PentaneC5H12 36.1
Hexane C6H1469
HeptaneC7H16 98.42
Octane C8H18125.52

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

Density and viscosity

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. 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.

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 = Mass/Volume

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, 10 ml and 100 ml 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 10 ml 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 3
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 100 ml 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 4
Solid Mass (g) Volume (ml) Density ( g·ml -1g·ml -1)
Solid 1      
Solid 2      
Solid 3      

Summary

  • The smallest unit of matter is the atom. Atoms can combine to form molecules.
  • A molecule is a group of two or more atoms that are attracted to each other by chemical bonds.
  • A small molecule consists of a few atoms per molecule. A giant molecule consists of millions of atoms per molecule, for example metals and diamonds.
  • The structure of a molecule can be represented in a number of ways.
  • The chemical formula of a molecule is an abbreviated way of showing a molecule, using the symbols for the elements in the molecule. There are two types of chemical formulae: molecular and empirical formula.
  • The molecular formula of a molecule gives the exact number of atoms of each element that are in the molecule.
  • The empirical formula of a molecule gives the relative number of atoms of each element in the molecule.
  • Molecules can also be represented using diagrams.
  • A ball and stick diagram is a 3-dimensional molecular model that uses 'balls' to represent atoms and 'sticks' to represent the bonds between them.
  • A space-filling model is also a 3-dimensional molecular model. The atoms are represented by spheres.
  • In a molecule, atoms are held together by chemical bonds or intramolecular forces. Covalent bonds, ionic bonds and metallic bonds are examples of chemical bonds.
  • A covalent bond exists between non-metal atoms. An ionic bond exists between non-metal and metal atoms and a metallic bond exists between metal atoms.
  • Intermolecular forces are the bonds that hold molecules together.
  • 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.
  • 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 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.

Summary exercise

  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.
    3. A composition of two or more atoms that act as a unit.
    4. Chemical formula that gives the relative number of atoms of each element that are in a molecule.
    Click here for the solution
  2. For each of the following questions, choose the one correct answer from the list provided.
    1. Ammonia, an ingredient in household cleaners, can be broken down to form one part nitrogen (N) and three parts hydrogen (H). This means that ammonia...
      1. is a colourless gas
      2. is not a compound
      3. cannot be an element
      4. has the formula N3H
      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. 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 5
    ElementMelting pointBoiling point (00C)
    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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