Introduction  (Page 3/3)

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 $\to$ 2Hg $+$ O ₂

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 $\to$ AB

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

2Mg + O ₂ $\to$ 2MgO

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

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 $\to$ 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 AgNO ₃ , 6M HCl, magnesium ribbon, iron filings, sulphur.

Note: AgNO ₃ 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 AgNO ₃ to the sodium chloride solution. NOTE: Please be careful AgNO ₃ 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: