Lesson Notes By Weeks and Term v5 - Grade 9

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

• Define and differentiate between physical and chemical change.
• Identify common signs of a chemical reaction.
• Explain what reaction rate means.
• Describe how temperature, concentration, and surface area affect reaction rate.
• Apply collision theory to explain these effects.

Lesson summary

Chemical reactions are the foundation of our world. From cooking food to the rusting of iron, chemical changes are happening all around us. Understanding these changes is not only crucial in science but also helps us understand everyday phenomena. In South Africa, understanding chemical processes can help us address issues like food spoilage, water purification, and the responsible use of resources. This week, we will begin our exploration of chemical change and how fast these changes occur – the rate of reaction. Understanding reaction rates can help us to prevent unwanted changes (like food going bad) and to speed up useful changes (like producing fertilisers).

Lesson notes

2.1 Physical vs. Chemical Change A physical change alters the form or appearance of a substance but does not change its chemical composition. For example, melting ice is a physical change because it is still water (H₂O), just in a different state. Boiling water, dissolving sugar in water, and crushing a can are all physical changes. A chemical change, also known as a chemical reaction, results in the formation of a new substance with different chemical properties. This involves the breaking and forming of chemical bonds. Examples of chemical changes include burning wood (producing ash, carbon dioxide, and water), rusting iron (producing iron oxide), and baking a cake (producing new compounds).

Key Difference: The key difference is whether new substances are formed. In a physical change, the substance remains chemically the same. In a chemical change, the original substance(s) are transformed into one or more different substances. 2.2 Indicators of a Chemical Change Identifying a chemical change can be tricky, but there are several common indicators: Colour Change: A dramatic and unexpected colour change often indicates a chemical reaction. For example, mixing two clear solutions that suddenly turn yellow or blue.

Gas Production: The formation of bubbles (gas) when two substances are mixed (and not simply boiling) usually indicates a chemical reaction. Think of adding vinegar to baking soda, producing carbon dioxide gas.

Precipitate Formation: When two clear solutions are mixed, and a solid (precipitate) forms, this is a strong indicator of a chemical reaction.

Temperature Change: Chemical reactions can release heat (exothermic reactions) or absorb heat (endothermic reactions). A noticeable temperature change (getting hotter or colder) during mixing suggests a chemical reaction.

Light Emission: Some reactions release light (e.g., burning).

Odour Change: Sometimes a new, distinct smell is produced by a reaction (e.g., the smell of something burning or rotting).

Example: Imagine you leave an apple on a table. Over time, it turns brown (colour change), the texture softens, and it might start to smell (odour change). This is due to chemical reactions occurring between the apple and oxygen in the air, leading to the formation of new compounds. 2.3 Reaction Rate The reaction rate is the speed at which a chemical reaction occurs. It tells us how quickly reactants are consumed and products are formed. A fast reaction rate means the reaction happens quickly, while a slow reaction rate means it happens slowly. We can measure the rate by monitoring how quickly reactants disappear or how quickly products appear.

Analogy: Think about a race between two cars. The speed of the car represents the reaction rate. The faster car completes the race quicker, just like a faster reaction consumes reactants quicker. 2.4 Factors Affecting Reaction Rate (Introduction) Several factors influence how fast a chemical reaction proceeds: Temperature: Generally, increasing the temperature increases the reaction rate. Think of cooking food – it cooks much faster at a higher temperature.

Concentration: Increasing the concentration of reactants increases the reaction rate. A higher concentration means more reactant molecules are available to react.

Surface Area: For reactions involving solids, increasing the surface area increases the reaction rate. Think of how quickly powdered sugar dissolves compared to a sugar cube.

Catalyst: A catalyst speeds up a reaction without being consumed in the reaction. Catalysts are very important in industrial processes to make them more efficient. (We will explore catalysts in more detail later) 2.5 Collision Theory (Introduction) The collision theory explains how reactions occur at a molecular level. For a reaction to occur, reactant molecules must: Collide: They must physically come into contact with each other.

Have Sufficient Energy: They must collide with enough kinetic energy (energy of motion) to break existing bonds and form new ones. This minimum energy is called the activation energy.

Have Correct Orientation: They must collide with the proper orientation to allow the necessary atoms to interact. How Collision Theory Explains Factors Affecting Rate: Temperature: Higher temperature means molecules move faster and have more kinetic energy. This leads to more frequent and more energetic collisions, increasing the reaction rate.

Concentration: Higher concentration means more reactant molecules in a given volume, leading to more frequent collisions and a higher reaction rate.

Surface Area: Increased surface area means more reactant molecules are exposed and available for collision, leading to more frequent collisions and a higher reaction rate. Guided Practice (With Solutions)

Question 1: Identify whether the following changes are physical or chemical: a) Dissolving salt in water. b) Burning a piece of paper. c) Freezing water into ice. d) Cooking an egg.

Solution: a) Physical.