Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define 'chemical reaction', 'reactant', and 'product'.
• Identify reactants and products in a reaction.
• Represent reactions using word equations.
• Distinguish between physical and chemical changes.
• Predict products of simple reactions.

Lesson summary

Chemical reactions are happening all around us, all the time! From cooking food in your kitchen to the rusting of iron in an old car, chemical reactions are transforming substances into new ones. Understanding chemical reactions is crucial for understanding how the world works. In South Africa, this knowledge is particularly important. For example, understanding how acids react with metals helps us manage acid mine drainage, a major environmental problem. Knowing about the chemical reactions in batteries is essential for using solar power 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. It's not just a change in appearance (like water freezing into ice – that's a physical change). A chemical reaction involves the breaking and forming of chemical bonds. Think of it like building something new with LEGO bricks. You take the original bricks apart (breaking bonds) and put them together in a new way to make something different (forming new bonds).

Reactants: Reactants are the substances that you start with in a chemical reaction. They are the "ingredients" that are mixed together and change. Reactants are always written on the left side of a chemical equation.

Products: Products are the new substances that are formed as a result of a chemical reaction. They are what you end up with. Products are always written on the right side of a chemical equation.

Word Equations: A word equation is a way of representing a chemical reaction using the names of the reactants and products. A word equation uses an arrow (→) to show the direction of the reaction. The arrow is read as "reacts to produce" or "yields." General Form of a Word Equation: Reactant 1 + Reactant 2 + ... → Product 1 + Product 2 + ... Examples of Chemical Reactions and Word Equations: Example 1: Burning Wood (Braai time!) When you burn wood for a braai, the wood (which is mostly carbon) reacts with oxygen in the air to produce carbon dioxide and water vapor. This reaction also releases heat and light, which is why we can cook over the fire.

Word Equation: Wood + Oxygen → Carbon Dioxide + Water Reactants: Wood, Oxygen Products: Carbon Dioxide, Water Example 2: Rusting of Iron (Old Cars!) When iron is exposed to oxygen and water (especially in humid coastal areas like Durban), it rusts. Rust is iron oxide.

Word Equation: Iron + Oxygen + Water → Iron Oxide (Rust)

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

Example 3: Baking a Cake (Birthday Celebration!) When you bake a cake, the ingredients (flour, sugar, eggs, etc.) react with each other in the heat to form a completely new substance – the cake! While not a single, simple chemical reaction, the overall process involves many chemical changes.

Simplified Word Equation: Cake Ingredients (Flour, Sugar, Eggs, etc.) + Heat → Cake Reactants: Cake Ingredients, Heat Product: Cake Example 4: Photosynthesis (Plant Life!) Plants use carbon dioxide from the air and water from the soil to produce glucose (sugar) and oxygen in the presence of sunlight. This process is called photosynthesis, and it's how plants make their own food.

Word Equation: Carbon Dioxide + Water + Sunlight → Glucose + Oxygen Reactants: Carbon Dioxide, Water, Sunlight Products: Glucose, Oxygen Example 5: Reaction of Vinegar and Baking Soda (Cleaning!) Vinegar (acetic acid) reacts with baking soda (sodium bicarbonate) to produce carbon dioxide gas, water, and sodium acetate. This is a common reaction used for cleaning or in baking.

Word Equation: Vinegar + Baking Soda → Carbon Dioxide + Water + Sodium Acetate Reactants: Vinegar, Baking Soda Products: Carbon Dioxide, Water, Sodium Acetate Physical Change vs. Chemical Change It’s important to understand the difference between physical and chemical changes.

Physical Change: A change that affects the form or appearance of a substance but does not change its chemical composition.

Examples: melting ice, boiling water, dissolving salt in water, cutting wood.

Chemical Change: A change that results in the formation of new substances with different chemical compositions.

Examples: burning wood, rusting iron, cooking an egg, the reaction between vinegar and baking soda. Using Chemical Symbols and Formulae While word equations are useful, chemists often use chemical symbols and formulae to represent reactions in a more concise way.

Chemical Symbols: Abbreviated representations of elements (e.g., H for Hydrogen, O for Oxygen, C for Carbon, Na for Sodium, Cl for Chlorine, Fe for Iron).

Chemical Formulae: Representations of molecules or compounds using chemical symbols and subscripts to indicate the number of atoms of each element (e.g., H2O for water, CO2 for carbon dioxide, O2 for oxygen, H2 for hydrogen, NaCl for sodium chloride [table salt], Fe2O3 for iron oxide [rust]). Using these, we can write chemical equations: Rusting of Iron (simplified): Fe + O2 → Fe2O3 (

Note: this equation needs to be balanced, which we will cover in later grades)

Formation of Water: H2 + O2 → H2O (Again, this needs to be balanced). Burning of Methane (main component of natural gas): CH4 + O2 → CO2 + H2O (Also needs balancing). Guided Practice (With Solutions)

Question 1: Identify the reactants and products in the following chemical reaction: Sugar + Oxygen → Carbon Dioxide + Water Solution: Reactants: Sugar, Oxygen Products: Carbon Dioxide, Water Explanation: Sugar and Oxygen are the substances that are reacting, so they are the reactants.