Heating Solids

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Resource Notes on Heating Solids

1. A solid is usually heated in a test tube or boiling tube made of heat-resistant material such as Pyrex. There are generally two ways of heating the solid.

a) The test tube is held using a test tube holder near the top of the tube and further away from the bottom of the tube as possible (see 1 below).

b) The test tube is clamped near its mouth to a retort stand. Heat is supplied to the bottom of the test tube (see 2 below). The test tube can either be clamped at a horizontal or slanted position.

Safety precautions during heating:

* Always wear safety goggles during the experiment, especially when heating

* Do not hold the test tube using bare hands as the test tube will turn very hot after some time.

* Always direct the mouth of the test tube toward the common walkway in the laboratory. Do not direct it toward yourself or to the person situated in front of you.

2. When using a Bunsen burner, switch to a moderate non-luminous (blue colour) flame. Always heat a solid gently unless otherwise instructed.

3. Notice droplets form due to water vapour condensing on the cooler part of the test tube.

4. There may be coloured / colourless fumes exiting the test tube mouth. It is a gas produced as a result of a chemical reaction involving the heating of the solid.

5. Testing of the gas can be carried out near the mouth of the test tube during heating. The procedures for testing different gases have been covered in the previous lesson. Refer to those worksheets if necessary.

6. Some solids may change colour upon heating. Record this colour change. After heating, allow the test tube to cool down to room temperature and observe any further colour change. Some solids may change colour on cooling. An example is zinc oxide.

Worksheet on Heating Solids

Worksheet title: To study the effects of heating different metal carbonates.

Apparatus: test tubes, delivery tube, retort stand with clamp, spatula, Bunsen burner

Chemicals: Calcium carbonate, sodium carbonate, zinc carbonate, limewater

Procedures

1. Add a spatula-full of calcium carbonate solid into a clean and dry test tube.

2. Fill another test tube with limewater to about full.

3. Set up the apparatus as shown in the diagram above.

4. Gently heat the solid with a medium-sized flame

5. Record any visible changes that you observe in both test tubes. Continue heating until no further change is observed.

6. Remove the heat and immediately raise the delivery tube from the limewater.

7. Let test tube to cool down to room temperature and note down any changes.

8. Repeat steps 1 to 5 with sodium carbonate and zinc carbonate.

9. Record all your observations in the table below.

Observations

Name of carbonate

Colour of carbonate at the start

Colour of solid left in tube

Observation in limewater

Before cooling

After cooling

calcium carbonate

sodium carbonate

zinc carbonate

(12m)

Questions

1. Based on your observation, which metal carbonate decomposes the fastest and which one decompose the slowest? (2m)

________________________________________________________________

2. Based on the record, what gas is produced during decomposition of carbonates? (1m)

________________________________________________________________

3. How can you relate the rate of decomposition of the metal carbonate to the reactivity series of metals? (1m)

________________________________________________________________

4. Write down a balanced chemical equation with state symbols for the decomposition of zinc oxide. (2m)

________________________________________________________________

Marking Scheme for the Worksheet on Heating Solids

Observations

Name of carbonate

Colour of carbonate at the start

Colour of solid left in tube

Observation in limewater

When hot

When cold

calcium carbonate

white

white

white

Turns chalky slowly

sodium carbonate

white

white

white

-

zinc carbonate

white

yellow

white

Turns chalky quickly

*1 mark for each correct answer in a box. (12m)

Questions

1. Based on your observation, which metal carbonate decomposes the fastest and which one decomposes the slowest?

Ans: Zinc carbonate decomposes fastest (1m); calcium carbonate decomposes slowest. (1m)

2. Based on the observation, what gas is produced during decomposition of carbonates?

Ans: Carbon dioxide (1m)

3. How can you relate the rate of decomposition of the metal carbonate to the reactivity series of metals?

Ans: The more reactive the metal, the harder it is to decompose its carbonate. (1m)

4. Write down a balanced chemical equation with state symbols for the decomposition of zinc oxide.

Ans: ZnCO3(s) -> ZnO(s) + CO2(g)

(1m for correct equation; 1m for correct state symbols)

References

Briggs, JGR (1999). Chemistry ‘O' Level Practical (2nd ed.). Longman.

Briggs, JGR and Heyworth, Rex M. (2007). Science in Focus Chemistry ‘O' Level Practical Workbook. Pearson Longman.

Hutchings, Kevin (2000). Classic Chemistry Experiments. Royal Society of Chemistry.

The Nuffield Foundation and Royal Society of Chemistry (2009). Practical Chemistry. Retrieved on November 22, 2009, from http://www.practicalchemistry.org/experiments/intermediate/oxidation-and-reduction/halogen-reactions-with-iron,44,EX.html

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