2.2 The properties of matter  (Page 3/3)

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.

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 forthe 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 atomsand '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 thechemical 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.
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 thatammonia...
      1. is a colourless gas
      2. is not a compound
      3. cannot be an element
      4. has the formula N ₃ H
   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.
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.
4. Refer to the table below which gives the melting and boiling points of a number of elements and then answer the questions thatfollow. ( Data from http://www.chemicalelements.com )

   Element	Melting point	Boiling point ( ${}^0$ C)
   copper	1083	2567
   magnesium	650	1107
   oxygen	-218.4	-183
   carbon	3500	4827
   helium	-272	-268.6
   sulphur	112.8	444.6

   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.