Science 7–10 · Years 9–10

Magnesium in acid: predicting the volume of hydrogen before you collect it

Reactions

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 reaction between a metal and an acid makes a salt and hydrogen, and the mass of metal fixes the volume of gas, so a prediction can be tested.

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 is an irritant
magnesium ribbon is flammable

Controls

no source of ignition anywhere in the room during collection
eye protection
collected hydrogen released outside or into a fume cupboard
acid neutralised before disposal

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

magnesium ribbon cleaned with emery paper, cut and weighed: a 0.04 g piece and a 0.10 g piece (lengths vary with ribbon thickness)
hydrochloric acid 1 mol/L, 50 mL per run (an excess for 0.10 g of magnesium)
100 mL conical flask with bung and delivery tube, trough, 250 mL measuring cylinder inverted over water (the 0.10 g piece gives about 104 mL, more than a 100 mL cylinder or gas syringe holds)
balance reading to 0.01 g, thermometer, stop clock

How to do it

Weigh both magnesium pieces and write the balanced equation Mg + 2HCl gives MgCl2 + H2.
Put 50 mL of 1 mol/L acid in the flask, fill the inverted cylinder with water in the trough and place the delivery tube under it.
Drop in the magnesium, bung the flask at once and collect the gas until bubbling stops; record the volume and the room temperature.
Before the second run, predict its volume from the first: with the acid in excess, volume is proportional to mass. Run it, then calculate the percentage difference between prediction and measurement.

What you should see

0.04 g of magnesium gives about 42 mL of gas and 0.10 g about 104 mL when collected over water at 25 °C and 101.3 kPa (hydrogen saturated with water vapour, 3.17 kPa at 25 °C; the dry hydrogen would be 40 mL and 101 mL); the volume is proportional to the mass. Measured volumes can differ by a few millilitres: gas lost before the bung is in and the thin oxide coat on the ribbon make them smaller, and gas still warm from the reaction makes them larger until it cools. The magnesium disappears completely and the flask warms. The learner knows it worked when the second run gives close to 2.5 times the first, matching the mass ratio.

What changes

What you change: mass of magnesium
What you measure: volume of hydrogen collected
What you keep the same: acid volume and concentration (in excess)
temperature
apparatus
time allowed for the reaction to finish

Common misconceptions

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

Doubling the acid volume doubles the gas, even when the acid is already in excess.
The metal dissolves like sugar rather than reacting.
The gas is air pushed out of the flask.

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)SC5-RXN-01 SC5-WS-06
Australian Curriculum v9AC9S9U07AC9S10U07

Sources

The pages the author read to write this activity.

All Concept Studio activities