Science 7–10 · Year 10

Lumps against powder: surface area and the rate of reaction on a balance

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

Only particles at a surface can collide with the acid, so the same mass of solid in smaller pieces reacts faster although the total gas made is the same.

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

hydrochloric acid 2 mol/L is corrosive to eyes
fine carbonate powder can froth over

Controls

eye protection
each solid is tipped in all at once, because adding the powder gradually would slow its start and spoil the comparison of initial gradients; with 40 mL of acid the 250 mL flask leaves more than 200 mL of space for froth below the plug
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. The quantities keep within CSIS Volume 2 Figure D-6 for Years 7 to 12: 200 mL of solution and 10 g of solid used in a reaction per student or group per activity.

What you need

calcium carbonate 2.00 g as large marble chips, as small chips and as powder, each weighed to 0.01 g (6.00 g in all, within the CSIS limit of 10 g of solid used in a reaction per student or group per activity)
hydrochloric acid 2 mol/L, 40 mL per run (a two-fold excess for 2.00 g of carbonate; the three runs use 120 mL, within the CSIS limit of 200 mL of solution per student or group per activity)
250 mL conical flask, cotton-wool plug, balance reading to 0.01 g, stop clock

How to do it

Put 40 mL of acid in the flask on the balance, tip in all the large chips at once, plug the neck with cotton wool and start the clock; record the mass every 30 seconds for 8 minutes.
Repeat with the small chips and with the powder, tipping each in all at once and using fresh acid each time.
Plot mass lost against time for the three runs on one set of axes.
Compare the initial gradients (rate at the start) and the final mass lost.

What you should see

2.00 g of calcium carbonate can release at most 0.88 g of carbon dioxide (about 0.49 L at 25 °C), and every run would reach that loss if it were followed to the end. Inside the 8 minutes recorded they do not: the powder run levels off first, the small chips next, and the large chips last, often still rising at 8 minutes. The initial gradient is steepest for the powder. The learner knows it worked when the three initial gradients rank powder, small chips, large chips, the powder run has flattened near a loss of 0.88 g before 8 minutes, and the large-chip curve is still climbing at 8 minutes at a smaller loss, so the three curves do not share an end point inside the recording window.

What changes

What you change: particle size of the calcium carbonate
What you measure: mass lost against time (rate from the gradient)
What you keep the same: mass of carbonate
acid volume and concentration
temperature
flask and plug

Common misconceptions

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

Powder makes more gas than lumps of the same mass.
The reaction stops when the fizzing is no longer loud.
A steeper line means a bigger reaction rather than a faster one.

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-02 SC5-WS-01
Australian Curriculum v9AC9S10U07AC9S10I03

Sources

The pages the author read to write this activity.

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