Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define and identify reactants and products.
• Distinguish between physical and chemical changes.
• Represent reactions using word equations.
• Explain the Law of Conservation of Mass.
• Provide everyday examples of chemical reactions in South Africa.

Lesson summary

Chemical reactions are fundamental processes that occur all around us, changing substances into new ones. From cooking food to the rusting of metal, chemical reactions are constantly shaping our world. Understanding what happens during a chemical reaction, specifically how reactants transform into products, is crucial for understanding everything from how we digest our food to how industries create essential materials. In South Africa, understanding chemical reactions can help us address challenges related to food security (understanding fertilizers), water purification (chemical treatments), and environmental protection (reducing pollution from industries).

Lesson notes

2.1 What is a Chemical Reaction? A chemical reaction is a process that involves the rearrangement of atoms and molecules to form new substances. These new substances have different properties compared to the original substances. A key sign of a chemical reaction is the formation of new substances, meaning that the chemical composition changes. This is different from a physical change, where the substance's form might change (e.g., ice melting into water), but the chemical composition remains the same (still H₂O). 2.2 Reactants and Products Reactants: These are the starting materials in a chemical reaction. They are the substances that undergo change. Think of them as the ingredients in a recipe.

Products: These are the substances formed as a result of the chemical reaction. They are the new substances that are created. Think of them as the dish you make from the recipe. In a chemical equation, reactants are written on the left-hand side, and products are written on the right-hand side, separated by an arrow (→) which represents "yields" or "produces". If there's more than one reactant or product, they are separated by a plus sign (+).

Example: Reactant A + Reactant B → Product C + Product D 2.3 Word Equations A word equation describes a chemical reaction using the names of the reactants and products. This is a simple way to represent what is happening in a reaction.

Example: Hydrogen + Oxygen → Water This means that hydrogen gas reacts with oxygen gas to produce water. 2.4 Physical vs. Chemical Changes | Feature | Physical Change | Chemical Change | |--------------------|----------------------------------------------------------|-----------------------------------------------------------| | Definition | Change in form or appearance, but not composition. | Change in chemical composition; new substances formed. | | Bonds | No bonds are broken or formed. | Bonds are broken and/or formed. | | Reversibility | Often easily reversible. | Often difficult to reverse. | | Examples | Melting ice, boiling water, dissolving sugar in water. | Burning wood, rusting iron, cooking an egg. | 2.5 The Law of Conservation of Mass This fundamental law states that matter cannot be created or destroyed in a chemical reaction. In other words, the total mass of the reactants must equal the total mass of the products. This means that the number of atoms of each element must be the same on both sides of the chemical equation. While we're not balancing equations yet, understand that this principle underpins that process. 2.6 Examples in South Africa Photosynthesis: Plants use sunlight, carbon dioxide, and water to produce glucose (sugar) and oxygen. This is essential for life and is happening in gardens, farms, and natural environments all over South Africa. Carbon dioxide + Water → Glucose + Oxygen.

Burning Wood (Braai): Wood reacts with oxygen during a braai to produce carbon dioxide, water, ash, and heat (energy). This is a common social activity in South Africa that demonstrates a chemical reaction. Wood + Oxygen → Carbon Dioxide + Water + Ash. Rusting of Iron (Ubiquitous in humid coastal areas): Iron reacts with oxygen and water (humidity) to form rust (iron oxide). This is a problem in coastal areas of South Africa, causing damage to infrastructure. Iron + Oxygen + Water → Rust (Iron Oxide). 2.7 Worked Examples Example 1: Burning of Methane (Natural Gas) Methane is the main component of natural gas, often used for cooking in South African households. When methane burns, it reacts with oxygen in the air.

Word Equation: Methane + Oxygen → Carbon Dioxide + Water This means methane reacts with oxygen to form carbon dioxide and water.

Example 2: Reaction of Vinegar (Acetic Acid) with Baking Soda (Sodium Bicarbonate) This is a common experiment and a component in some cleaning mixtures.

Word Equation: Acetic Acid + Sodium Bicarbonate → Sodium Acetate + Water + Carbon Dioxide This means that the acetic acid in vinegar reacts with sodium bicarbonate (baking soda) to produce sodium acetate (a salt), water, and carbon dioxide gas (which causes the fizzing).

Example 3: The Haber Process (Ammonia Production) This is a key industrial process for producing ammonia, used in fertilizers that are essential for agriculture in South Africa.

Word Equation: Nitrogen + Hydrogen → Ammonia This means that nitrogen gas reacts with hydrogen gas to produce ammonia gas. Guided Practice (With Solutions)

Question 1: Identify the reactants and products in the following word equation: Magnesium + Oxygen → Magnesium Oxide Solution: Reactants: Magnesium and Oxygen Product: Magnesium Oxide

Commentary: This is a straightforward application of the definitions. The substances on the left side of the arrow are the reactants, and the substance on the right side is the product.

Question 2: Is the following change a physical change or a chemical change? Explain your answer. Dissolving salt in water.

Solution: This is a physical change.