Science 7–10 · Year 8

Making and testing three gases: hydrogen, oxygen and carbon dioxide

Change

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

A gas produced in a reaction is a new substance, and each gas has a test that identifies it.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

hydrogen is extremely flammable
hydrochloric acid 1 mol/L irritates skin and eyes
hydrogen peroxide irritates eyes; manganese(IV) oxide is harmful if inhaled
limewater is an irritant

Controls

no flames on the bench while hydrogen is being collected; the adult performs the splint tests away from the generator
eye protection throughout
neutralise acid residues before disposal
never mix manganese(IV) oxide with metal powders

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson.

What you need

three boiling tubes with bungs and delivery tubes, trough of water, three test tubes for collection over water
magnesium ribbon weighed to about 0.04 g (3 to 4 cm; ribbon thickness varies) and hydrochloric acid 1 mol/L, 20 mL
hydrogen peroxide 3 percent w/v (10 volume), 20 mL, and manganese(IV) oxide, a pinch on a spatula tip
marble chips (calcium carbonate), 3 to 4 chips, and hydrochloric acid 1 mol/L, 20 mL
limewater 5 mL, wooden splints, matches (adult), eye protection

How to do it

Hydrogen: add the magnesium to 20 mL of acid in a boiling tube, fit the delivery tube and collect the gas over water in an inverted test tube; keep the tube mouth-down until tested.
Oxygen: put a pinch of manganese(IV) oxide in a boiling tube, add 20 mL of 3 percent hydrogen peroxide, fit the delivery tube and collect over water.
Carbon dioxide: add the marble chips to 20 mL of acid and collect over water in the same way.
Adult tests: lit splint at the mouth of the hydrogen tube; glowing splint into the oxygen tube; bubble carbon dioxide through limewater or shake the collected gas with 5 mL of limewater.
Record each observation and write a word equation for the reaction that made each gas.

What you should see

Hydrogen burns with a squeaky pop; 0.04 g of magnesium ribbon gives about 42 mL of hydrogen collected over water at 25 °C and 101.3 kPa (40 mL as dry gas), but the first gas out is the air pushed from the boiling tube, so the collected tube holds hydrogen mixed with air, which still gives the pop. The glowing splint relights in oxygen; 20 mL of 3 percent hydrogen peroxide can release up to 216 mL of oxygen. Limewater turns cloudy white with carbon dioxide within seconds. The learner knows it worked when all three tests give the expected sign and a control tube of air gives none of them.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Fizzing always means carbon dioxide.
The pop test is the sound of the tube cracking.
Oxygen itself burns.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC4-CHG-01 SC4-WS-01
Australian Curriculum v9AC9S8U07AC9S8I03

Sources

The pages the author read to write this activity.

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