Year 10 Science | Victorian Curriculum 2.0
Chemical reactions & conservation of mass
Topic 04 | Chemical sciences | Answer key

Year 10 answers

Fluency

Word and symbol equations

    1. In a chemical reaction, the total mass of reactants equals the total mass of products — atoms are rearranged, not created or destroyed.
    2. Hydrogen + oxygen →\to water.
    3. 2H2O→2H2+O22\text{H}_2\text{O} \to 2\text{H}_2 + \text{O}_2.
    4. Reactants: zinc, hydrochloric acid. Products: zinc chloride, hydrogen.
    5. There are 22 water molecules (so 44 H atoms and 22 O atoms in total from that term).
Fluency

Balancing

    1. H2+Cl2→2HCl\text{H}_2 + \text{Cl}_2 \to 2\text{HCl}.
    2. 4Na+O2→2Na2O4\text{Na} + \text{O}_2 \to 2\text{Na}_2\text{O}.
    3. 4Al+3O2→2Al2O34\text{Al} + 3\text{O}_2 \to 2\text{Al}_2\text{O}_3.
    4. Mg+2HCl→MgCl2+H2\text{Mg} + 2\text{HCl} \to \text{MgCl}_2 + \text{H}_2.
    5. 2C2H6+7O2→4CO2+6H2O2\text{C}_2\text{H}_6 + 7\text{O}_2 \to 4\text{CO}_2 + 6\text{H}_2\text{O}.
    6. 2Fe2O3+3C→4Fe+3CO22\text{Fe}_2\text{O}_3 + 3\text{C} \to 4\text{Fe} + 3\text{CO}_2.
Fluency

Conservation of mass

    1. 1616 g.
    2. 10−5.6=4.410 - 5.6 = 4.4 g.
    3. 5−3=25 - 3 = 2 g.
    4. 4.6+3.55=8.154.6 + 3.55 = 8.15 g.
    5. 28.6−20=8.628.6 - 20 = 8.6 g.
Reasoning

Reaction types and explain

    1. (a) Synthesis. (b) Decomposition. (c) Displacement. (d) Combustion.
    2. Subscripts describe what the molecule is. Changing them changes the compound (water to hydrogen peroxide). Only coefficients (whole-number multiples of molecules) may be changed.
    3. No. Iron combines with oxygen from the air; the added mass is oxygen. Mass of iron + oxygen consumed = mass of rust formed.
    4. Mass was converted to CO2_2 and water vapour that escaped into the air. If the candle burned in a sealed system, no mass would be lost.
    5. 88+36=12488 + 36 = 124 g total products.
Problem solving

Real reactions

    1. CO2_2 mass: 5−3.2=1.85 - 3.2 = 1.8 g. Equation: CuCO3→CuO+CO2\text{CuCO}_3 \to \text{CuO} + \text{CO}_2.
    2. Zn+CuSO4→ZnSO4+Cu\text{Zn} + \text{CuSO}_4 \to \text{ZnSO}_4 + \text{Cu}. Displacement (Zn is more reactive than Cu).
    3. 2C4H10+13O2→8CO2+10H2O2\text{C}_4\text{H}_{10} + 13\text{O}_2 \to 8\text{CO}_2 + 10\text{H}_2\text{O}.
    4. 2Na+2H2O→2NaOH+H22\text{Na} + 2\text{H}_2\text{O} \to 2\text{NaOH} + \text{H}_2.
Reasoning

Challenge

    1. Perform the reaction in a closed, weighed flask fitted with a balloon or sealed gas collection. The total mass of the sealed system before and after the reaction will be equal. Alternatively, measure the masses of CaO produced and CO2_2 collected separately and sum them.
    2. Balanced equation: 2H2+O2→2H2O2\text{H}_2 + \text{O}_2 \to 2\text{H}_2\text{O}. 3.03.0 g H2_2 needs 24.024.0 g O2_2 (stoichiometric ratio 4:32=1:84 : 32 = 1 : 8), so both are fully consumed. Mass of water =3.0+24.0=27.0= 3.0 + 24.0 = 27.0 g. The ampoule contains only water (as liquid/vapour).
    3. C2H5OH+3O2→2CO2+3H2O\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \to 2\text{CO}_2 + 3\text{H}_2\text{O}. Check: C 2=22 = 2; H 6=66 = 6; O 1+6=2+31 + 6 = 2 + 3, i.e. 7=77 = 7. Balanced.
    4. Open vessels allow gases to enter or leave, making “before” and “after” masses disagree. Examples: (i) a burning candle — mass appears to decrease as CO2_2 and water vapour escape; (ii) iron rusting in open air — mass appears to increase as it absorbs oxygen from the atmosphere.
Year 10 Science study companion | Answer key