Science 7–10 · Years 9–10
Heating metal carbonates: which ones break down, and by how much
Reactions
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 decomposition reaction turns one compound into simpler substances; heating some metal carbonates breaks them down into a metal oxide and carbon dioxide, and how easily they break down follows the reactivity of the metal.
Safety card
Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
Hazards
- copper(II) carbonate is harmful
- hot tubes; suck-back can crack a hot tube
- limewater is an irritant
Controls
- eye protection
- delivery tube removed from the limewater before the flame is removed
- copper and zinc residues to labelled waste
- lead carbonate is not used
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
- about 2 g each of copper(II) carbonate (basic), zinc carbonate, calcium carbonate and sodium carbonate, each weighed to 0.01 g
- four borosilicate test tubes with bungs and delivery tubes, limewater 5 mL per tube, test-tube rack
- Bunsen burner, test-tube holder, heat-resistant mat, balance reading to 0.01 g, stop clock
How to do it
- Weigh each carbonate in its test tube. Fit the delivery tube so it dips into limewater.
- Heat each tube strongly for the same time (3 minutes), watching for a colour change and for the limewater turning cloudy; record the time the cloudiness first appears.
- Lift the delivery tube out of the limewater before removing the heat (to prevent suck-back). Cool and reweigh each tube.
- Calculate the percentage of its starting mass each carbonate lost, and rank the carbonates by how quickly they clouded the limewater and how much mass they lost.
What you should see
Copper carbonate turns from green to black within a minute and clouds the limewater fastest. Zinc carbonate turns yellow while hot and white when cool. Calcium carbonate needs strong heating and loses only a little mass in 3 minutes; sodium carbonate shows no change and no cloudiness. For the teacher: complete decomposition would lose 28.1 percent of the mass of school copper carbonate, which is the basic salt Cu2(OH)2CO3 (the simple formula CuCO3 would lose 35.6 percent), 35.1 percent of pure zinc carbonate or 25.9 percent of the basic salt Zn5(CO3)2(OH)6 that school zinc carbonate often is, and 44.0 percent of calcium carbonate. The learner knows it worked when copper and zinc carbonates clearly lose mass and cloud the limewater within the 3 minutes, calcium carbonate loses little, sodium carbonate loses nothing, and the ranking by limewater time is copper, zinc, calcium, sodium.
What changes
- What you change
- metal carbonate
- What you measure
- time to cloud the limewater and percentage mass lost
- What you keep the same
- mass of carbonate
- heating time and flame
- limewater volume
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Heating a solid only melts it.
- The mass lost is the substance burning away.
- All carbonates behave the same when heated.
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-01SC5-WS-06
- Australian Curriculum v9AC9S10U07AC9S9U07
Sources
The pages the author read to write this activity.
- curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
- edu.rsc.org/experiments/thermal-decomposition-of-metal-carbonates/450.article
- edu.rsc.org/experiments/thermal-decomposition-of-calcium-carbonate/704.article
- education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/5._Volume_2_Appendices.pdf