Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define a chemical reaction.
• Identify reactants and products in a chemical reaction.
• Explain the Law of Conservation of Mass.
• Differentiate between physical and chemical changes.
• Describe the role of energy in chemical reactions.

Lesson summary

Chemical reactions are the backbone of everything around us. From the food we eat to the air we breathe, chemical reactions are constantly taking place. Understanding what reactants and products are is crucial to understanding how the world works and how we can use chemistry to solve problems. In South Africa, understanding chemical reactions is especially important for various industries like mining, agriculture, and manufacturing. Knowing how different substances interact allows us to create new materials, improve farming techniques, and manage our resources more effectively.

Lesson notes

What is a Chemical Reaction? A chemical reaction is a process that involves the rearrangement of atoms and molecules to form new substances. This rearrangement results in changes in the chemical properties of the substances involved. We can represent a chemical reaction using a chemical equation.

Reactants and Products: Reactants: These are the substances that start a chemical reaction. They are the "ingredients" that react with each other. We write them on the left side of the arrow in a chemical equation.

Products: These are the new substances that are formed as a result of the chemical reaction. We write them on the right side of the arrow in a chemical equation.

Example 1: Burning Wood When we burn wood in a fire (like a braai!), the wood reacts with oxygen in the air.

Reactants: Wood (mainly carbon compounds) + Oxygen Products: Carbon dioxide + Water + Heat (energy)

Word Equation: Wood + Oxygen → Carbon dioxide + Water Example 2: Rusting of Iron The rusting of iron is a chemical reaction where iron reacts with oxygen and water to form rust (iron oxide). This is a significant issue in South Africa, especially in coastal areas with high humidity, affecting infrastructure and vehicles.

Reactants: Iron + Oxygen + Water Product: Rust (Iron Oxide)

Word Equation: Iron + Oxygen + Water → Rust Example 3: Baking a Cake Baking a cake involves many chemical reactions. For instance, baking powder (sodium bicarbonate) decomposes in the presence of heat and moisture.

Reactant: Baking powder Products: Carbon dioxide + Water + Sodium Carbonate Word Equation: Baking powder → Carbon dioxide + Water + Sodium Carbonate The Arrow (→): The arrow in a chemical equation means "reacts to produce" or "yields." It shows the direction of the reaction.

The Law of Conservation of Mass: One of the most important principles in chemistry is the Law of Conservation of Mass. This law states that in a closed system, the total mass of the reactants must equal the total mass of the products. In other words, matter cannot be created or destroyed in a chemical reaction; it only changes form. Atoms are rearranged but their total number remains the same.

Example: If you burn 10g of wood and it reacts with 20g of oxygen, the total mass of the carbon dioxide, water, and ash produced will be approximately 30g. (

Note: some mass is lost as gases escape) Physical Change vs.

Chemical Change: It's important to distinguish between a physical change and a chemical change.

Physical Change: A change that affects the appearance or form of a substance, but not its chemical composition. Examples include melting ice (water is still water), cutting paper, dissolving sugar in water.

Chemical Change: A change that results in the formation of a new substance with different chemical properties. Examples include burning wood (wood becomes ash and gases), rusting iron (iron becomes rust), cooking an egg (the egg white changes its structure). A useful mnemonic is to remember signs of a chemical reaction are often 'TEARS': Temperature change, Effervescence (bubbles), Appearance of a precipitate, Reactant disappears, Smell/odor change.

Energy in Chemical Reactions: Chemical reactions involve energy changes.

Exothermic Reactions: Reactions that release energy in the form of heat or light. The products have less energy than the reactants. Burning wood and explosions are exothermic reactions.

Endothermic Reactions: Reactions that absorb energy from their surroundings. The products have more energy than the reactants. Photosynthesis (plants using sunlight to create food) is an endothermic reaction.