Chemical Reactions and Equations — Evidence, Balancing and Patterns · Learner pages

Lesson 1 · Read evidence and chemical equations

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

Name or initials: ____________   Date: ____________

A physical change can alter shape or state without making a different substance.

1. Record the observation. Write what a person could observe, such as a new solid or a temperature change. A statement such as “a reaction happened” is a conclusion, not an observation.
2. Check for new substances. A change of colour, gas formation, a precipitate or temperature change can support a reaction claim. These signs are not proof by themselves: boiling water also makes bubbles. Use the identities and other evidence supplied.
3. Read the equation. In N₂ + 3H₂ → 2NH₃, nitrogen and hydrogen are reactants and ammonia is the product. The arrow reads “produces”. The coefficient 3 counts three H₂ molecules in a particle model. It does not change H₂ into a different substance.
4. Include state and conditions. N₂(g) + 3H₂(g) → 2NH₃(g) labels gases. (s), (l), (g) and (aq) mean solid, liquid, gas and dissolved in water. Conditions over an arrow are required conditions, not additional products. This industrial reaction is a reading example, not an activity to perform.

Worked example: A supplied record says: ice becomes liquid water; a second record identifies a new insoluble substance after two solutions react. The first is a physical state change because the substance is still H₂O. The second supports chemical change because a new substance is identified. Observation and substance identification together support the conclusion.

1. Which entry is an observation?

   1. The reaction is certainly oxidation. · 2. A white solid appeared. · 3. A new chemical must have formed.

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

   1. (g) · 2. (aq) · 3. (l)

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

   1. H₂ · 2. NH₃ · 3. N₂

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.

Lesson 2 · Balance by conserving atom counts

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

Name or initials: ____________   Date: ____________

A subscript belongs to a chemical formula; a coefficient multiplies the whole formula.

1. Keep chemical formulas fixed. Start with the supplied formulas Al + O₂ → Al₂O₃. Count Al: 1 on the left and 2 on the right; O: 2 on the left and 3 on the right. Do not edit subscripts to force equality: that would describe different substances.
2. Find a common oxygen count. The smallest shared multiple of 2 and 3 is 6. Place 3 before O₂ and 2 before Al₂O₃. Oxygen now totals 6 on each side. The product side now has 2 × 2 = 4 aluminium atoms.
3. Match aluminium. Place 4 before Al: 4Al + 3O₂ → 2Al₂O₃. A coefficient acts on every element in its formula. In solid aluminium oxide, the drawing represents composition in formula units, not isolated Al₂O₃ molecules.
4. Audit and simplify. Check Al: 4 = 4 and O: 3 × 2 = 2 × 3 = 6. The coefficients 4, 3 and 2 share no whole-number factor greater than 1, so the ratio is simplest. Equal atom counts express conservation; total molecule counts need not match.

Worked example: Balance N₂ + H₂ → NH₃. Two N atoms on the left require 2NH₃ on the right. That creates six H atoms on the right, so use 3H₂. N₂ + 3H₂ → 2NH₃ has N: 2 = 2 and H: 6 = 6. Coefficients 1:3:2 are the smallest whole-number ratio.

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

   1. 3 · 2. 2 · 3. 6

2. Which equation is balanced?

   1. 2Al + O₂ → Al₂O₃ · 2. Al + O₂ → Al₂O₃ · 3. 4Al + 3O₂ → 2Al₂O₃

3. Which change is allowed while balancing supplied formulas?

   1. Changing a coefficient before a formula. · 2. Changing O₂ into O₃ to fit a count. · 3. Deleting an element from a product.

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.

Lesson 3 · Recognise combination and decomposition

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

Name or initials: ____________   Date: ____________

A coefficient tells quantity; it does not create an extra kind of substance.

1. Identify distinct substances. Read formulas on each side and count kinds of substances, not coefficient totals. In a symbolic pattern A + B → AB there are two reactant kinds and one product kind. A and B are placeholders, not actual chemical formulas.
2. Recognise combination. N₂ + 3H₂ → 2NH₃ is a combination reaction because nitrogen and hydrogen form the single product substance ammonia. The coefficient 2 does not mean two different products. Actual chemical conditions still matter.
3. Recognise decomposition. 2H₂O₂ → 2H₂O + O₂ is decomposition: one reactant substance produces water and oxygen. Depending on the substance, decomposition may be driven by heat, light or electricity, or aided by a catalyst. Read the stated conditions; do not infer that every decomposition needs the same trigger.
4. Keep classifications distinct. Combination and decomposition describe the substance pattern. Exothermic and endothermic describe energy transfer. An equation may receive a pattern label and an energy label when evidence supports both; the pattern alone is not an energy measurement.

Worked example: A supplied reaction is 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O when heated. It has one reactant kind and three product kinds, so it is decomposition. Check Na: 2 = 2, H: 2 = 2, C: 2 = 1 + 1, O: 6 = 3 + 2 + 1. This is a paper classification example, not heating instructions.

1. Which pattern represents combination?

   1. A + B → AB · 2. AB → A + B · 3. AB + CD → AD + CB

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

   1. 3 · 2. 1 · 3. 2

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

   1. Decomposition · 2. Combination · 3. No chemical change

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.

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 separate oxygen molecules. / Three hydrogen molecules, containing six hydrogen atoms. / One molecule containing three hydrogen atoms.

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

   Conclude that every warming change is combustion. / Replace the measurement with a guess. / Check conditions and other evidence before explaining the rise.

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

   6 / 4 / 2

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

   A balanced equation does not conserve matter. / Atoms of each element, rather than molecule counts, must be conserved. / Atoms may disappear during every reaction.

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

   All combination reactions use nitrogen. / Two reactant substances form one product substance. / The right-hand coefficient is even.

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

   One reactant substance forms three product substances. / There are three sodium atoms. / The products have no oxygen.

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?

   Ammonia is a reactant because it is named last. / The arrow means all substances are unchanged. / Nitrogen and hydrogen are starting substances.

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

   Its shape never changes. / It must release oxygen gas. / Its chemical identity remains H₂O.

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

   1 : 2 : 1 / 1 : 1 : 2 / 2 : 1 : 1

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

   Unbalanced because the coefficients are even. / Replace Al₂O₃ with AlO. / Balanced, but divide all coefficients by 2 for the simplest ratio.

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

   Only that one product is formed. / Only that a coefficient is 2. / Evidence that energy is released to surroundings.

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

   They are rearranged while their totals are conserved. / Every element loses half its atoms. / New element types must appear.