Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define and distinguish between chemical and physical changes.
• Identify reactants and products in a chemical reaction.
• Explain what 'rate of reaction' means.
• Describe how temperature, concentration, and surface area affect reaction rates.

Lesson summary

Chemical reactions are happening all around us, all the time! From the food we cook to the batteries that power our cell phones, chemical changes are essential to modern life. Even the rusting of metal gates in your yard and the souring of milk are examples of chemical changes. Understanding these changes, and how quickly they happen (the rate of reaction), is crucial for many industries, including agriculture, manufacturing, and even medicine. Imagine being able to predict how quickly a fertilizer will break down in the soil to nourish crops or how long a medication will remain effective!

Lesson notes

2.1 Chemical vs. Physical Changes It's crucial to understand the difference between physical and chemical changes.

Physical Change: A physical change alters the form or appearance of a substance but does not change its chemical composition. The molecules of the substance remain the same.

Examples: Melting ice: Water is still water, just in a different state.

Dissolving salt in water: Salt particles are dispersed among water molecules, but they are still salt.

Cutting wood: You still have wood, just in smaller pieces.

Chemical Change: A chemical change involves the formation of new substances with different chemical compositions. The molecules of the original substances are rearranged to form new molecules. This often involves breaking and forming chemical bonds.

Examples: Burning wood: Wood reacts with oxygen to produce ash, carbon dioxide, water, and other substances. The ash is not wood.

Rusting of iron: Iron reacts with oxygen and water to form iron oxide (rust), which is a different substance than iron.

Cooking an egg: The proteins in the egg undergo a chemical change (denaturation) and coagulate, changing the egg's texture and properties.

Photosynthesis: Plants take in carbon dioxide and water and, using sunlight, convert them into glucose (sugar) and oxygen. How to tell the difference?

Indicators of a chemical change include: Change in color Formation of a gas (bubbles) Formation of a precipitate (a solid forming in a liquid solution) Change in temperature (heat released or absorbed) A new substance with different properties is formed. 2.2 Reactants and Products In a chemical reaction, the substances that react with each other are called reactants. The substances that are formed as a result of the reaction are called products. For example, consider the burning of methane gas (natural gas, often used for cooking and heating in South Africa): Methane + Oxygen → Carbon Dioxide + Water Here: Reactants: Methane and Oxygen Products: Carbon Dioxide and Water We can represent this as a chemical equation: CH 4 + 2O 2 → CO 2 + 2H 2 O 2.3 Rate of Reaction The rate of reaction refers to how quickly a chemical reaction happens. It can be thought of as the speed at which reactants are converted into products. A fast reaction has a high rate, while a slow reaction has a low rate. 2.4 Factors Affecting Rate of Reaction Several factors can influence how quickly a chemical reaction proceeds.

We'll focus on three key factors: Temperature: Increasing the temperature generally increases the rate of reaction. At higher temperatures, molecules have more kinetic energy and move faster. This means they collide more frequently and with greater force, increasing the likelihood of successful reactions. For example, food spoils faster at room temperature than in a refrigerator because the bacteria causing the spoilage reproduce (a chemical reaction) faster at higher temperatures.

Another example: It is harder to start a fire in the winter. The colder wood burns slower.

Concentration: Concentration refers to the amount of a substance in a given volume. Increasing the concentration of reactants generally increases the rate of reaction. With more reactant molecules present, there are more frequent collisions, leading to a higher reaction rate. Imagine trying to light a candle with just a tiny puff of oxygen versus blowing hard. The higher concentration of oxygen in the stronger breath will help the candle light faster or burn stronger.

Surface Area: For reactions involving solids, increasing the surface area generally increases the rate of reaction. A larger surface area means that more reactant molecules are exposed and available to react. Consider burning a log versus wood shavings. The wood shavings, with their much larger surface area exposed to oxygen, will catch fire much faster.

Another common example: Sugar dissolves faster in water if it is granulated (smaller particles, larger surface area) than if it's in a solid block. Think of dissolving brown sugar (larger pieces) vs. white sugar. Example Scenario (Using South African Context): Imagine a farmer who wants to use fertilizer to help their crops grow. The fertilizer contains chemicals that react with the soil to release nutrients for the plants.

Temperature: In the hot summer months, the reaction between the fertilizer and the soil might happen faster than in the cooler winter months. This means the nutrients are released more quickly in summer.

Concentration: If the farmer uses a higher concentration of fertilizer (more fertilizer per unit of soil), the reaction will likely happen faster, releasing more nutrients more quickly.

However, using too much fertilizer can also be harmful.

Surface Area: If the fertilizer is in the form of small granules (larger surface area), it will react more quickly with the soil than if it's in large clumps.