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Science 7–10 · Year 10

Synthesis, decomposition and displacement: three reactions, three patterns

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

Reactions fall into patterns that can be read from their equations: two substances joining, one splitting, or one element taking the place of another.

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

  • burning magnesium: intense light and hot ash
  • copper(II) carbonate and copper(II) sulfate are harmful
  • hot test tube

Controls

  • magnesium burnt by the teacher only, viewed from the side
  • eye protection
  • copper residues to labelled waste

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

  • synthesis (teacher demonstration): 5 cm magnesium ribbon, tongs, Bunsen burner, heat-resistant mat
  • decomposition: 1 g copper(II) carbonate in a test tube with delivery tube into 5 mL limewater
  • displacement: 5 mL copper(II) sulfate 0.5 mol/L and a 2 cm strip of zinc or an iron nail in a test tube
  • eye protection, test-tube rack, hand lens

How to do it

  1. Watch the teacher burn the magnesium ribbon; record the reactants and the product and write the equation, then balance it.
  2. Heat the copper carbonate until the limewater clouds and the solid is black; write and balance the equation.
  3. Stand the zinc strip in the copper sulfate for 10 minutes; record the changes to the metal and the solution; write and balance the equation.
  4. For each reaction, draw the particles before and after and label the pattern: synthesis (A + B gives AB), decomposition (AB gives A + B) or displacement (A + BC gives AC + B).
  5. Classify four further equations from the textbook by pattern and predict their products.

What you should see

Synthesis: the ribbon burns with a white flame to a white powder, 2Mg + O2 gives 2MgO. Decomposition: green powder turns black and the limewater clouds, CuCO3 gives CuO + CO2 (basic copper carbonate also gives water). Displacement: the zinc darkens with a brown-pink coating and the blue solution fades, Zn + CuSO4 gives ZnSO4 + Cu. The learner knows it worked when each observation matches its pattern and the atom counts balance on both sides of every equation.

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.

  • Every reaction is a kind of burning.
  • The products of a reaction are arbitrary and must be memorised one by one.
  • Atoms are created or destroyed when a compound decomposes.

Curriculum references

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

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  2. edu.rsc.org/experiments/thermal-decomposition-of-metal-carbonates/450.article
  3. edu.rsc.org/experiments/displacement-reactions-between-metals-and-their-salts/720.article
  4. edu.rsc.org/balanced-chemical-equations/the-change-in-mass-when-magnesium-burns/718.article

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