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15. Chemistry and electricityTopic

Acids and alkalis, electrolysis of salt solution.

Timing

3045 min.

Description

Students make up a salt solution with indicator and complete an electrical circuit. Thecations / anions are attracted to the carbon electrode causing the indicator to change

colour.

Apparatus and equipment (per group)

Plastic petri dish

Filter papers

6 V battery or power pack

Leads and crocodile clips

Carbon electrode

Dropping pipette. Use the type of teat pipette (usually fitted to Universal Indicatorbottles) that does not allow squirting egGriffin.

Chemicals (per group)

Sodium chloride

Universal Indicator

Methyl orange.

Teaching tips

Other indicators to try might include: bromocresol green (lead attached to positiveterminal), screened methyl orange (try both terminals), blue litmus (positive) and redlitmus (negative). Phenolphthalein does not work very well in this experiment.

Background theory

When the pencil is attached to the negative lead, H+

ions are attracted to it,producing the colour associated with acids for that particular indicator. If the pencil is

attached to the positive lead, the reverse happens.

Safety

Wear eye protection.

Answers

1. When attached to the negative lead the writing is red, when attached to thepositive lead it is purple.

2. H+

ions are attracted to the negative electrode, OH

ions are attracted to thepositive electrode. So depending on which electrode the pencil is attached to it willaffect the colour of the indicator and therefore the writing.

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Chemistry and electricityIntroduction

In this experiment, electricity and some indicators are used to make coloured writing.

What to do

1. Dissolve a spatula measure of sodium chloride in 2 cm3

of water. Add three dropsof methyl orange indicator.

2. Lay a filter paper inside a plastic petri dish. Drop the solution onto the paper usinga dropping pipette, until the paper holds no more solution.

3. Attach the positive end of a 6 V battery to a lead ending in a crocodile clip. Use thecrocodile clip to grip one end of the paper.

4. Attach the negative end of the battery to a carbon electrode.

5. Write lightly on the wet paper, using the carbon electrode. What colour is thewriting?

6. Repeat the experiment using Universal Indicator. Describe what happens.

Safety

Wear eye protection.

Questions

1. What would happen if the lead were attached to the positive electrode using

Universal Indicator? Try this if there is time.

2. Explain what reactions have occurred to produce the colours.

38 Classic chemistry experiments

Filter paper soaked with

sodium chloride and

indicator solution

Petri dish

Carbon electrode

Negative terminal

LeadBattery or

power pack

Positive terminal

6 V

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16. CombustionTopic

Combustion.

Timing

30 min.

Description

A candle is burnt inside a gas jar. The students then test for the presence of carbondioxide and water.

Apparatus and equipment (per group)

Gas jar and lid

Candle on a tray

Heat-proof mat.

Chemicals (per group)

Limewater 0.02 mol dm3

Blue cobalt chloride paper.

Teaching tips

As an extension, the students could suggest other experiments to do to find out if other

fuels form carbon dioxide and water when they burn.

Data logging sensors and software can be used to demonstrate what may be happeningin the jar as the candle burns. Use a bell jar and place sensors inside to monitorhumidity, temperature, light or oxygen levels as the candle burns. The software willshow the changes as a graph against time. When the candle extinguishes, readmit airto the jar and continue to record for a few moments.

Safety

Wear eye protection. Some fuels might be quite hazardous. A thorough risk assessmentshould be done.

Answers

1. Oxygen.2. Carbon dioxide and water.

3. Methane or similar hydrocarbon or fuel.

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