Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Differentiate between physical and chemical changes.
• Identify reactants and products in a reaction.
• Write balanced chemical equations.
• Describe common chemical reactions in everyday life.
• Predict if a change is physical or chemical.

Lesson summary

Chemical changes are fundamental to understanding how the world around us functions. From the burning of fuels in our cars to the cooking of food in our kitchens, chemical changes are constantly occurring and transforming matter. In South Africa, understanding chemical changes is crucial for industries ranging from mining and agriculture to the development of new medicines and environmentally sustainable practices. For instance, the extraction of gold from ore relies heavily on chemical reactions, and understanding these reactions can improve efficiency and reduce environmental impact.

Lesson notes

2.1 Physical Change vs. Chemical Change A physical change is a change in the form or appearance of a substance, but not in its chemical composition. This means the molecules of the substance are rearranged, but not broken apart or reformed into new molecules. Key indicators of a physical change include: Change in state (solid, liquid, gas).

Examples: Melting ice (H₂O(s) → H₂O(l)), boiling water (H₂O(l) → H₂O(g)). Change in size or shape.

Examples: Crushing a can, dissolving sugar in water (though dissolving can sometimes be accompanied by a small temperature change, making it tricky). The sugar remains sugar even when dissolved. Mixture formation (without chemical reaction).

Examples: Mixing sand and water. A chemical change, also known as a chemical reaction, involves the rearrangement of atoms and molecules to form new substances with different chemical compositions. Evidence of a chemical change often includes: Change in color (not just dilution). Formation of a precipitate (a solid that forms from a solution). Production of a gas (bubbles). Release or absorption of heat (exothermic or endothermic reaction). Production of light. Irreversibility (usually, though some chemical reactions are reversible).

Examples: Burning wood, rusting iron (Fe(s) + O₂(g) → Fe₂O₃(s)), baking a cake. 2.2 Representing Chemical Change: Chemical Equations Chemical reactions are represented by chemical equations. A chemical equation uses chemical symbols and formulas to show the reactants (starting materials) and products (substances formed).

Reactants: Substances that undergo change. Written on the left side of the equation.

Products: Substances that are formed. Written on the right side of the equation. Arrow (→): Indicates the direction of the reaction ("reacts to form" or "yields"). Plus sign (+): Separates multiple reactants or products.

State symbols: Indicate the physical state of the substance: (s) - solid, (l) - liquid, (g) - gas, (aq) - aqueous (dissolved in water).

Example: The reaction of methane (CH₄) with oxygen (O₂) to produce carbon dioxide (CO₂) and water (H₂O) is a combustion reaction.

The unbalanced equation is: CH₄(g) + O₂(g) → CO₂(g) + H₂O(g) 2.3 Balancing Chemical Equations A balanced chemical equation has the same number of atoms of each element on both sides of the equation. This follows the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a chemical reaction. Balancing is achieved by adding coefficients (numbers in front of the chemical formulas).

Steps for Balancing Chemical Equations: Write the unbalanced equation. Count the number of atoms of each element on both sides of the equation. Start balancing with the element that appears in only one reactant and one product. Use coefficients to adjust the number of atoms of each element until they are equal on both sides. Balance polyatomic ions (if present) as a single unit if they appear unchanged on both sides. Check your work to ensure that the equation is balanced. Ensure coefficients are in the simplest whole-number ratio. Example (Balancing the Methane Combustion Equation): Unbalanced equation: CH₄(g) + O₂(g) → CO₂(g) + H₂O(g)

Count atoms: Reactants: C = 1, H = 4, O = 2 Products: C = 1, H = 2, O = 3 Balance hydrogen: Multiply H₂O by 2: CH₄(g) + O₂(g) → CO₂(g) + 2H₂O(g)

Count atoms (revised): Reactants: C = 1, H = 4, O = 2 Products: C = 1, H = 4, O = 4 Balance oxygen: Multiply O₂ by 2: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

Count atoms (final): Reactants: C = 1, H = 4, O = 4 Products: C = 1, H = 4, O = 4 Balanced equation: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) 2.4 Common Chemical Reactions Combustion: A rapid reaction between a substance with an oxidant, usually oxygen, to produce heat and light.

Example: Burning coal (mostly carbon) in a power plant: C(s) + O₂(g) → CO₂(g)

Rusting (Corrosion): The oxidation of a metal, often iron, in the presence of oxygen and water.

Example: Rusting of a metal roof: 4Fe(s) + 3O₂(g) + 6H₂O(l) → 4Fe(OH)₃(s) (simplified)

Acid-Base Reactions: Reactions between an acid and a base, resulting in the formation of a salt and water.

Example: Neutralization of stomach acid (HCl) with an antacid (containing magnesium hydroxide, Mg(OH)₂): 2HCl(aq) + Mg(OH)₂(s) → MgCl₂(aq) + 2H₂O(l)

Precipitation Reactions: Reactions where two aqueous solutions combine to form an insoluble solid (precipitate).

Example: Formation of lead iodide precipitate: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq) Guided Practice (With Solutions)

Question 1: Classify the following changes as either physical or chemical changes, and briefly explain your reasoning: a) Sugar dissolving in water. b) Burning firewood. c) Melting butter. d) Iron nail rusting. e) Crushing a rock.

Solution: a)

Physical Change: The sugar molecules are dispersing throughout the water, but they are still sugar molecules. No new substance is formed.