Experiment: Combustion of alcohols

The combustion of alcohols is a Form 5 practical in the Carbon Compounds chapter that shows how a homologous series burns and what products form. It reinforces the idea that organic fuels release energy and give carbon dioxide and water, and it prepares the ground for the separate experiment that measures the heat of combustion. This guide sets out the aim, the apparatus, a numbered procedure, the observations to expect, and the science process skills the practical papers reward.

Aim

To investigate the combustion of alcohols in air, to identify the products of complete combustion, and to compare how cleanly different members of the alcohol series burn.

Apparatus and materials

• Small spirit burners or crucibles with wicks
• A short series of alcohols, for example methanol, ethanol, propan-1-ol and butan-1-ol
• Dry boiling tube or funnel connected to limewater
• Anhydrous copper(II) sulfate (white) or cobalt(II) chloride paper (blue) to test for water
• Limewater to test for carbon dioxide
• Filter funnel, delivery tube and suction (a filter pump) to draw combustion products through the tests
• Tongs, heatproof mat, and a white tile

Procedure

1. Set up a small spirit burner containing the first alcohol and light it carefully.
2. Hold a cool, dry boiling tube (or an inverted funnel) briefly above the flame so that any liquid product condenses on the cool glass.
3. Test the condensed liquid: add it to white anhydrous copper(II) sulfate, or touch it to blue cobalt(II) chloride paper, and record the colour change.
4. Using a filter pump, draw the gases from above the burning alcohol through a delivery tube into a test tube of limewater and observe the limewater.
5. Observe and record the colour and sootiness of the flame, and hold a white tile briefly in the flame to see whether soot is deposited.
6. Extinguish the burner, let it cool, then repeat steps 1 to 5 with each alcohol in turn, keeping the apparatus the same.
7. Record for each alcohol the flame appearance, whether water forms, and whether the limewater changes.

Expected observations

Each alcohol burns with a flame and gives off heat, and the apparatus above the flame becomes warm. A colourless liquid condenses on the cool glass; it turns white anhydrous copper(II) sulfate blue (or turns blue cobalt(II) chloride paper pink), showing the liquid is water. The gas drawn through limewater turns the limewater milky (cloudy white), showing carbon dioxide is produced. As you move along the series to alcohols with more carbon atoms, the flame tends to become more yellow, more luminous and sootier, and more soot is deposited on the white tile.

Inference and conclusion

Alcohols undergo combustion: they react with oxygen in the air, releasing heat, and in complete combustion the products are carbon dioxide and water. The water is confirmed by turning anhydrous copper(II) sulfate blue, and the carbon dioxide by turning limewater milky. The increasing sootiness along the series shows that alcohols with a higher proportion of carbon are more likely to burn incompletely, depositing carbon (soot) when the air supply is limited. The conclusion is that alcohols are combustible fuels whose complete combustion gives carbon dioxide and water, while incomplete combustion produces soot.

Science process skills (Paper 3 style)

Making an inference. From the observation that the gas turns limewater milky, infer that carbon dioxide is a product of the combustion.

Identifying variables. If the investigation compared sootiness along the series, the manipulated variable is the alcohol used (its number of carbon atoms), the responding variable is how sooty the flame is (soot deposited on a tile), and the controlled variables are the air supply, the wick and the distance of the tile from the flame.

Classifying. Classify each product test by what it detects: the water test (anhydrous copper(II) sulfate or cobalt chloride paper) detects water; limewater detects carbon dioxide.

Communicating. Tabulate the results with a row for each alcohol and columns for flame colour, soot, water test and limewater test, so a trend along the series can be read at a glance.

Safety precautions

• Wear safety goggles, because the alcohols are flammable and burn with an almost invisible flame in some cases.
• Keep the alcohol bottles stoppered and well away from the flame, so the vapour does not catch fire.
• Light only a small amount of alcohol in a proper burner, so the flame stays controlled.
• Use tongs to hold the boiling tube and tile in the flame, to avoid burns from the hot glass.
• Work on a heatproof mat in a ventilated area, so heat and combustion gases are handled safely.

Common errors

• Using a wet tube for the water test. If the tube is already damp, you cannot tell whether the water came from the flame; start with a dry, cool tube.
• Confusing the two product tests. Anhydrous copper(II) sulfate turning blue shows water, not carbon dioxide; limewater turning milky shows carbon dioxide. Do not swap them.
• Not keeping conditions the same. Comparing sootiness is only fair if the wick, air supply and tile position are the same for each alcohol; change only the alcohol.
• Reporting heat values. This experiment identifies the products and compares flames; do not invent numerical heat readings here, which belong to the heat-of-combustion experiment.
• Missing incomplete combustion. A yellow, sooty flame is itself a result; record it rather than adjusting the air until every flame looks the same.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we make sure students can name the products of combustion, state the test for each, and explain why sootiness increases along the series, all answers the written and practical papers reward. Combustion of alcohols connects the reactions of carbon compounds with energy changes across SPM Chemistry, and it leads directly into the separate heat-of-combustion practical and the Paper 3 practical assessment (Paper 3 is a practical test assessing science process skills), where clear product tests and fair comparisons score the marks.