Chemical Reactions and Equations — Evidence, Balancing and Patterns · Answer guide

Lesson 1 · Read evidence and chemical equations

Separate an observation from a conclusion, then identify reactants, products and physical states.

1. Which entry is an observation?

   Answer: A white solid appeared.. The visible solid is an observation; the other statements interpret a cause or reaction type.

2. Which symbol means a substance is dissolved in water?

   Answer: (aq). Aqueous, (aq), means dissolved in water; (l) means the liquid state.

3. In N₂ + 3H₂ → 2NH₃, which substance is the product?

   Answer: NH₃. NH₃ is on the right of the reaction arrow.

Discuss: Paper task: make two columns headed Observation and Inference. Sort “temperature rose by 5 °C” and “energy was released” into them, then state what extra information would strengthen the inference. Do not mix household chemicals.

Teaching note: Bubbles alone do not prove that a new gas was chemically made; a liquid can boil without changing its chemical identity. Open responses need discussion; they are not automatically marked.

Lesson 2 · Balance by conserving atom counts

Balance a supplied equation with the smallest whole-number coefficients and verify every element.

1. How many oxygen atoms are represented by 3O₂?

   Answer: 6. Three groups each contain two oxygen atoms: 3 × 2 = 6.

2. Which equation is balanced?

   Answer: 4Al + 3O₂ → 2Al₂O₃. The correct equation has Al 4 on each side and O 6 on each side.

3. Which change is allowed while balancing supplied formulas?

   Answer: Changing a coefficient before a formula.. Coefficients alter quantity while keeping substance identity fixed.

Discuss: Use labelled paper counters for atoms. Rearrange the same counters from one side to the other; record the count of each element before and after. The paper model does not demonstrate real reaction conditions or rate.

Teaching note: Changing Al₂O₃ into AlO changes the specified compound. Balancing changes coefficients, never the chemical formulas supplied. Open responses need discussion; they are not automatically marked.

Lesson 3 · Recognise combination and decomposition

Classify reaction patterns by the number of different reactant and product substances.

1. Which pattern represents combination?

   Answer: A + B → AB. Multiple reactant substances form one product substance in combination.

2. How many product substances occur in N₂ + 3H₂ → 2NH₃?

   Answer: 1. Only ammonia occurs on the product side; its coefficient is 2.

3. Which label fits 2H₂O₂ → 2H₂O + O₂?

   Answer: Decomposition. One starting substance forms two different product substances.

Discuss: On paper, sort supplied equation cards by pattern. Add a separate “energy evidence not supplied” column so classification does not become a guess about temperature.

Teaching note: Two NH₃ molecules are still one product substance. Do not count a coefficient as the number of different substances. Open responses need discussion; they are not automatically marked.

Final check · Set 1

Try one set after the lessons. Use the other set after reviewing any errors.

1. What does 3H₂ represent in a particle-count model?

   Three hydrogen molecules, containing six hydrogen atoms.. Each H₂ molecule contains two hydrogen atoms, so three contain six.

2. A record says “the measured temperature changed from 22 °C to 27 °C”. Which is the best next scientific step?

   Check conditions and other evidence before explaining the rise.. External heating and other causes must be considered before making a chemical explanation.

3. For 2CO + O₂ → 2CO₂, how many oxygen atoms are on each side?

   4. Left: 2 from 2CO plus 2 from O₂; right: 2 × 2 = 4.

4. Why can a balanced reaction have unequal total molecule counts on its two sides?

   Atoms of each element, rather than molecule counts, must be conserved.. Atoms rearrange into molecules, so molecule numbers may change while atom totals remain equal.

5. Which is a valid reason to call N₂ + 3H₂ → 2NH₃ a combination reaction?

   Two reactant substances form one product substance.. Classification follows substance pattern, not coefficient parity or a specific element.

6. Why is 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O decomposition?

   One reactant substance forms three product substances.. There is only NaHCO₃ on the left and three distinct formulas on the right.

Final check · Set 2

Try one set after the lessons. Use the other set after reviewing any errors.

1. Which equation reading is correct for nitrogen + hydrogen → ammonia?

   Nitrogen and hydrogen are starting substances.. Substances before the arrow are reactants.

2. Why is melting ice a physical change in the supplied example?

   Its chemical identity remains H₂O.. Ice and liquid water have the same chemical formula; the state changes.

3. Which simplest ratio balances H₂ + Cl₂ → HCl?

   1 : 1 : 2. One H₂ and one Cl₂ supply two H and two Cl atoms, making 2HCl.

4. A learner gives 8Al + 6O₂ → 4Al₂O₃. What is the best feedback?

   Balanced, but divide all coefficients by 2 for the simplest ratio.. Both sides have Al 8 and O 12, but all coefficients share factor 2.

5. What extra evidence is needed to label an unfamiliar reaction exothermic?

   Evidence that energy is released to surroundings.. Energy classification depends on energy transfer, not merely equation pattern.

6. In a balanced decomposition, what happens to the atoms of each element?

   They are rearranged while their totals are conserved.. Chemical reactions rearrange atoms without changing element identities.