Classic chemistry experiments
15.
37
Chemistry and electricity
Topic
Acids and alkalis, electrolysis of salt solution.
Timing
30–45 min.
Description
Students make up a salt solution with indicator and complete an electrical circuit. The
cations / 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 Indicator
bottles) that does not allow squirting – eg Griffin.
Chemicals (per group)
▼ Sodium chloride
▼ Universal Indicator
▼ Methyl orange.
Teaching tips
Other indicators to try might include: bromocresol green (lead attached to positive
terminal), screened methyl orange (try both terminals), blue litmus (positive) and red
litmus (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 the
positive lead it is purple.
+
–
2. H ions are attracted to the negative electrode, OH ions are attracted to the
positive electrode. So depending on which electrode the pencil is attached to it will
affect the colour of the indicator and therefore the writing.
38
Classic chemistry experiments
Chemistry and electricity
Introduction
In this experiment, electricity and some indicators are used to make coloured writing.
Filter paper soaked with
sodium chloride and
indicator solution
Petri dish
Positive terminal
Carbon electrode
Negative terminal
6V
Battery or
power pack
Lead
What to do
3
1. Dissolve a spatula measure of sodium chloride in 2 cm of water. Add three drops
of methyl orange indicator.
2. Lay a filter paper inside a plastic petri dish. Drop the solution onto the paper using
a 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 the
crocodile 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 the
writing?
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.
Classic chemistry experiments
39
16. Combustion
Topic
Combustion.
Timing
30 min.
Description
A candle is burnt inside a gas jar. The students then test for the presence of carbon
dioxide and water.
Apparatus and equipment (per group)
▼ Gas jar and lid
▼ Candle on a tray
▼ Heat-proof mat.
Chemicals (per group)
–3
▼ Limewater 0.02 mol dm
▼ 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 happening
in the jar as the candle burns. Use a bell jar and place sensors inside to monitor
humidity, temperature, light or oxygen levels as the candle burns. The software will
show the changes as a graph against time. When the candle extinguishes, readmit air
to the jar and continue to record for a few moments.
Safety
Wear eye protection. Some fuels might be quite hazardous. A thorough risk assessment
should be done.
Answers
1. Oxygen.
2. Carbon dioxide and water.
3. Methane or similar hydrocarbon or fuel.