Matter is all around us. The desks we sit at, the air we breathe and the water we drink are all examples of matter. But matter doesn't always stay the same. It can change in many different ways. In this chapter, we are going to take a closer look at physical and chemical changes that occur in matter.

A physical change is one where the particles of the substances that are involved in the change are not broken up in any way. When water is heated for example, the temperature and energy of the water molecules increases and the liquid water evaporates to form water vapour. When this happens, some kind of change has taken place, but the molecular structure of the water has not changed. This is an example of a physical change.

H₂O (l)→→ H₂O (g)

Conduction (the transfer of energy through a material) is another example of a physical change. As energy is transferred from one material to another, the energy of each material is changed, but not its chemical makeup. Dissolving one substance in another is also a physical change.

Definition 1: Physical change

A change that can be seen or felt, but that doesn't involve the break up of the particles in the reaction. During a physical change, the form of matter may change, but not its identity. A change in temperature is an example of a physical change.

You can think of a physical change as a person who is standing still. When they start to move (start walking) then a change has occurred and this is similar to a physical change.

There are some important things to remember about physical changes in matter:

When a chemical change takes place, new substances are formed in a chemical reaction. These new products may have very different properties from the substances that were there at the start of the reaction.

The breakdown of copper(II) chloride to form copper and chlorine is an example of chemical change. A simplified diagram of this reaction is shown in Figure 2. In this reaction, the initial substance is copper(II) chloride, but once the reaction is complete, the products are copper and chlorine.

Figure 2: The decomposition of copper(II) chloride to form copper and chlorine. We write this as: CuCl₂→→Cu++Cl₂

Definition 2: Chemical change

The formation of new substances in a chemical reaction. One type of matter is changed into something different.

There are some important things to remember about chemical changes:

We will consider two types of chemical reactions: decomposition reactions and synthesis reactions.

Decomposition reactions

A decomposition reaction occurs when a chemical compound is broken down into elements or smaller compounds. The generalised equation for a decomposition reaction is:

AB →→ A + B

One example of such a reaction is the decomposition of hydrogen peroxide (Figure 3) to form hydrogen and oxygen according to the following equation:

2H₂O₂ →→ 2H₂O ++ O₂

Figure 3: The decomposition of H₂O₂ to form H₂O and O₂

The decomposition of mercury (II) oxide is another example.

Experiment : The decomposition of mercury (II) oxide

Aim

To observe the decomposition of mercury (II) oxide when it is heated.

Note: Because this experiment involves mercury, which is a poisonous substance, it should be done in a fume cupboard, and all the products of the reaction must be very carefully disposed of.

Apparatus

Mercury (II) oxide (an orange-red product); two test tubes; a large beaker; stopper and delivery tube; Bunsen burner; wooden splinter.

Figure 4

Method

1. Put a small amount of mercury (II) oxide in a test tube and heat it gently over a Bunsen burner. Then allow it to cool. What do you notice about the colour of the mercury (II) oxide?
2. Heat the test tube again and note what happens. Do you notice anything on the walls of the test tube? Record these observations.
3. Test for the presence of oxygen by holding a glowing splinter in the mouth of the test tube.

Results

1. During the first heating of mercury (II) oxide, the only change that took place was a change in colour from orange-red to black and then back to its original colour.
2. When the test tube was heated again, deposits of mercury formed on the inner surface of the test tube. What colour is this mercury?
3. The glowing splinter burst into flame when it was placed in the test tube, meaning that oxygen is present.

Conclusions

When mercury oxide is heated, it decomposes to form mercury and oxygen. The chemical decomposition reaction that takes place can be written as follows:

2HgO →→ 2Hg ++ O₂

Synthesis reactions

During a synthesis reaction, a new product is formed from elements or smaller compounds. The generalised equation for a synthesis reaction is as follows:

A + B →→ AB

One example of a synthesis reaction is the burning of magnesium in oxygen to form magnesium oxide(Figure 5). The equation for the reaction is:

2Mg + O₂→→ 2MgO

Figure 5: The synthesis of magnesium oxide (MgO) from magnesium and oxygen

Experiment: Chemical reactions involving iron and sulphur

Aim

To demonstrate the synthesis of iron sulphide from iron and sulphur.

Apparatus

5,6 g iron filings and 3,2 g powdered sulphur; porcelain dish; test tube; Bunsen burner

Figure 6

Method

1. Measure the quantity of iron and sulphur that you need and mix them in a porcelain dish.
2. Take some of this mixture and place it in the test tube. The test tube should be about 1/3 full.
3. This reaction should ideally take place in a fume cupboard. Heat the test tube containing the mixture over the Bunsen burner. Increase the heat if no reaction takes place. Once the reaction begins, you will need to remove the test tube from the flame. Record your observations.
4. Wait for the product to cool before breaking the test tube with a hammer. Make sure that the test tube is rolled in paper before you do this, otherwise the glass will shatter everywhere and you may be hurt.
5. What does the product look like? Does it look anything like the original reactants? Does it have any of the properties of the reactants (e.g. the magnetism of iron)?

Results

1. After you removed the test tube from the flame, the mixture glowed a bright red colour. The reaction is exothermic and produces energy.
2. The product, iron sulphide, is a dark colour and does not share any of the properties of the original reactants. It is an entirely new product.

Conclusions

A synthesis reaction has taken place. The equation for the reaction is:

Fe + S →→ FeS

Investigation : Physical or chemical change?

Apparatus:

Bunsen burner, 4 test tubes, a test tube rack and a test tube holder, small spatula, pipette, magnet, a birthday candle, NaCl (table salt), 0,1M AgNO₃, 6M HCl, magnesium ribbon, iron filings, sulphur.

Note: AgNO₃ stains the skin. Be careful when working with it or use gloves.

Method:

1. Place a small amount of wax from a birthday candle into a test tube and heat it over the bunsen burner until it melts. Leave it to cool.
2. Add a small spatula of NaCl to 5 ml water in a test tube and shake. Then use the pipette to add 10 drops of AgNO₃ to the sodium chloride solution. NOTE: Please be careful AgNO₃ causes bad stains!!
3. Take a 5 cm piece of magnesium ribbon and tear it into 1 cm pieces. Place two of these pieces into a test tube and add a few drops of 6M HCl. NOTE: Be very careful when you handle this acid because it can cause major burns.
4. Take about 0,5 g iron filings and 0,5 g sulphur. Test each substance with a magnet. Mix the two samples in a test tube and run a magnet alongside the outside of the test tube.
5. Now heat the test tube that contains the iron and sulphur. What changes do you see? What happens now, if you run a magnet along the outside of the test tube?
6. In each of the above cases, record your observations.

Questions:

Decide whether each of the following changes are physical or chemical and give a reason for your answer in each case. Record your answers in the table below:

Table 1

Description	Physical or chemical change	Reason
melting candle wax
dissolving NaCl
mixing NaCl with AgNO3
tearing magnesium ribbon
adding HCl to magnesium ribbon
mixing iron and sulphur
heating iron and sulphur