Physical ScienceChemical ChangeYear 9 Science

Tests for hydrogen, oxygen, carbon dioxide and water

What this lesson covers

Why invisible gases need chemical tests
The pop test for hydrogen
The glowing splint test for oxygen
The limewater test for carbon dioxide
The cobalt chloride paper test for water
Natural indicators such as red cabbage for sorting acids and alkalis
Safe handling of corrosive substances

Key idea

Why we need tests

Hydrogen, oxygen and carbon dioxide are all colourless and have no smell, so you cannot tell them apart by looking or sniffing.
A standard test is a reaction with a clear, agreed result that only one substance gives.
Because everyone uses the same test, a result in an Auckland lab means the same as a result in a Dunedin one.

Substance	Test	Positive result
Hydrogen gas	Hold a lit splint at the mouth of the tube	Squeaky pop
Oxygen gas	Put a glowing splint into the tube	Splint relights
Carbon dioxide gas	Bubble the gas through limewater	Limewater turns milky
Water (liquid)	Touch with dry blue cobalt chloride paper	Paper turns pink

Key idea

Hydrogen: the pop test

A simple way to make hydrogen is to drop a piece of magnesium ribbon into dilute hydrochloric acid: magnesium + hydrochloric acid → magnesium chloride + hydrogen.
Hydrogen is lighter than air, so collect it in a test tube held upside down.
Bring a lit splint to the mouth of the tube. The hydrogen burns very quickly with the oxygen in the air and makes a squeaky pop.
The product of that tiny explosion is water: hydrogen + oxygen → water.

Magnesium ribbon fizzing in acid, with an upside-down test tube over it collecting hydrogen. That tube is then held mouth down over a lit splint, which gives a squeaky pop.

Key idea

Oxygen: the glowing splint test

Oxygen can be made by adding a pinch of manganese dioxide to hydrogen peroxide; the manganese dioxide speeds up the breakdown without being used up.
Light a wooden splint, then blow it out so only the tip glows.
Lower the glowing splint into the tube. If it bursts back into flame, the gas is oxygen.
Air is only about 21 percent oxygen, which is not enough to relight the splint; nearly pure oxygen is needed.

Key idea

Carbon dioxide: the limewater test

Carbon dioxide forms when an acid reacts with a carbonate, such as marble chips in hydrochloric acid or baking soda in vinegar.
Limewater is a clear solution of calcium hydroxide in water.
Bubble the gas through it with a delivery tube. If the limewater turns cloudy or milky, the gas is carbon dioxide.
The cloudiness is tiny grains of solid calcium carbonate: carbon dioxide + calcium hydroxide → calcium carbonate + water.
A lit splint goes out in carbon dioxide, but it also goes out in nitrogen, so that is not a reliable test on its own.

Marble chips in hydrochloric acid in a stoppered conical flask. A delivery tube carries the gas into a test tube of limewater, where it bubbles through and turns the limewater milky.

Key idea

Water: cobalt chloride paper

Cobalt chloride paper is blue when completely dry and turns pink when it touches water.
Dry the paper in a warm oven or keep it sealed with a drying agent, otherwise moisture in the air turns it pink before you start.
A positive result shows that water is present, not that the liquid is pure water; sea water and orange juice also turn it pink.
To show a liquid is pure water, you also measure its boiling point (100 °C at sea level) and its freezing point (0 °C).
Cobalt chloride is toxic, so handle the paper with tweezers and wash your hands afterwards.

Worked example

Which gas came off the zinc?

A student adds zinc granules to dilute sulfuric acid and collects the bubbles. A glowing splint does not relight. A lit splint gives a squeaky pop. Identify the gas.

Glowing splint does not relightso it is not oxygen
Lit splint makes a squeaky popthe positive result for hydrogen
Metal + acid fitsreactive metals and acids give off hydrogen
The gas is hydrogen

Worked example

Is there carbon dioxide in your breath?

You breathe out gently through a straw into limewater for a minute. Air from a pump is bubbled through a second tube of limewater for the same time. What result shows your breath contains extra carbon dioxide?

Pumped air: limewater stays clear or turns cloudy very slowlyair has only about 0.04 percent carbon dioxide
Breath: limewater turns milky within a minuteexhaled air has about 4 percent carbon dioxide
Same time and same volume of limewatera fair comparison
Breath contains much more carbon dioxide than airyour cells make it in respiration

Key idea

Making a natural pH indicator

Many plants contain coloured chemicals that change colour with pH, so you can make an indicator in a kitchen.
Chop some red cabbage, cover it with hot water for about ten minutes, then strain off the purple liquid.
Add a few drops to each test liquid in a clean, clear container and compare the colours against a white background.
To check your colour chart, test something you already know: vinegar for acid, tap water for roughly neutral, and baking soda solution for alkali.
Turmeric works too, but it only changes from yellow to red-brown in alkalis, so it cannot tell an acid from a neutral liquid.

Liquid	Red cabbage colour	Turmeric colour	Conclusion
Lemon juice	Red to pink	Yellow	Acidic
Vinegar	Pink	Yellow	Acidic
Distilled water	Purple	Yellow	Neutral
Baking soda solution	Blue to green	Orange-red	Alkaline
Washing soda solution	Green to yellow	Red-brown	Strongly alkaline

Red cabbage indicator colours along the pH scale from 1 to 14: red and pink for acids, purple around neutral pH 7, then blue, green and yellow for alkalis.

Key idea

Handling corrosive substances safely

A corrosive substance destroys skin and damages metal; containers show a hazard symbol of liquid dripping onto a hand and a bar of metal.
Wear safety glasses for any acid or alkali, even a dilute one, because an eye is damaged far faster than skin.
When diluting, add the acid slowly to water, never water to concentrated acid, because the heat released can make it spit.
If any splashes onto skin or into an eye, rinse it with running water for at least 15 minutes and tell a teacher or get medical help.
At home, never mix bleach with acidic toilet cleaners or with ammonia cleaners, because both mixtures release toxic gases.
In New Zealand workplaces, every hazardous chemical comes with a safety data sheet that lists its dangers, the protective gear to wear and the first aid to give.

Setting	Example corrosive	Key safety steps
School lab	Dilute hydrochloric acid	Safety glasses, small volumes, wipe spills, wash hands
Home	Drain or oven cleaner	Gloves, ventilation, never mix products, store away from children
Workplace	Concentrated sulfuric acid	Read the safety data sheet, face shield and gloves, eyewash station nearby

Common mistakes

A lit splint went out in the gas, so it must be carbon dioxide.
A splint also goes out in nitrogen and other gases, so only limewater turning milky confirms carbon dioxide.
Hydrogen relights a glowing splint, and oxygen pops with a lit one.
It is the other way round: hydrogen gives a squeaky pop with a lit splint, and oxygen relights a glowing one.
The liquid turned cobalt chloride paper pink, so it is pure water.
Pink only shows water is present; a pure sample also has a boiling point of exactly 100 °C at sea level.
To dilute an acid, pour the water into the acid.
Add the acid slowly to the water, because water poured into concentrated acid heats up fast and can spit acid out.

Try it at home

Identify four mystery white powders

You need: salt, sugar, baking soda, cornflour, vinegar, 4 small cups or saucers, a teaspoon, a helper

Your helper puts half a teaspoon of each powder into a cup and labels the cups only A, B, C and D, keeping a secret list of which is which.
Water test: add 2 teaspoons of cold water to half of each sample and stir. Three dissolve to a clear liquid; one stays cloudy and settles to the bottom. That one is cornflour.
Vinegar test: add a few drops of vinegar to the other half of each sample. One fizzes as carbon dioxide forms. That one is baking soda.
Crystal test: put a few drops of the two remaining clear liquids on a dark plate and leave them on a windowsill overnight. Salt leaves small square crystals; sugar leaves a sticky film.
Look for: each test sorts the powders by one property, so together they name all four without tasting any; check your answers against your helper's list.

Safety: Never identify a powder by tasting or sniffing it, even in a kitchen, and keep vinegar splashes away from your eyes.

Your turn

1. A gas relights a glowing splint. What is it?
2. Marble chips are added to hydrochloric acid. Which test would you use on the gas, and what result do you expect?
3. A clear liquid turns blue cobalt chloride paper pink. Does this prove the liquid is pure water? Explain.
4. Red cabbage indicator turns blue-green in a cleaning spray. Is the spray acidic, neutral or alkaline?
5. A student spills dilute sodium hydroxide on their hand. What should they do first?

Show answer

1. Oxygen; only nearly pure oxygen relights a glowing splint. 2. Bubble it through limewater; the limewater turns milky because the gas is carbon dioxide from the acid and carbonate. 3. No. It shows water is present, but the liquid could have other things dissolved in it; you would also check its boiling point is 100 °C. 4. Alkaline, since red cabbage turns blue to green above pH 7. 5. Rinse the hand under running water for at least 15 minutes and tell the teacher; sodium hydroxide is corrosive even when dilute.

Summary

Recap

Hydrogen gives a squeaky pop with a lit splint.
Oxygen relights a glowing splint.
Carbon dioxide turns limewater milky as calcium carbonate forms.
Water turns dry blue cobalt chloride paper pink, though that shows water is present, not that it is pure.
Plant dyes such as red cabbage act as indicators; check them with known acids and alkalis first.
Treat acids and alkalis with eye protection, dilute acid into water, never mix cleaning products, and rinse any splash for 15 minutes.

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