Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define chemical change vs. physical change with examples.
• Identify reactants and products in a reaction.
• Define 'rate of reaction' and factors that affect it.
• Recognize the common signs of a chemical reaction.
• Write simple word equations for chemical reactions.

Lesson summary

Chemical changes are fundamental to life as we know it. They are happening all around us, from the food we digest to the fuels that power our vehicles. Understanding chemical changes and how fast they occur (the rate of reaction) is crucial for many fields, including medicine, agriculture, and industry. In South Africa, this knowledge is particularly important. For example, optimizing the fermentation process in the production of traditional beer or improving the efficiency of fertilizer production for our agricultural sector both rely on understanding chemical reactions.

Lesson notes

What is a Chemical Change? A chemical change, also known as a chemical reaction, is a process that involves the rearrangement of atoms and molecules to form new substances. This means the chemical composition of the original substance(s) is altered. Unlike physical changes, which only change the appearance or state of a substance (like melting ice or dissolving sugar in water), chemical changes create entirely new substances with different properties.

Reactants: The substances that start a chemical reaction. They are the "ingredients." Products: The new substances formed as a result of the chemical reaction.

Signs of a Chemical Change: How do we know a chemical change has occurred?

Look for these tell-tale signs: Change in colour: For example, iron rusting (turning reddish-brown). Think of the corrugated iron sheets used extensively in informal settlements; when they rust, that's a chemical change.

Formation of a gas (effervescence): Bubbles appearing when you mix baking soda and vinegar. This is relevant in baking bread in South African households.

Formation of a precipitate: A solid forming when two solutions are mixed. This is similar to the process of water purification where solid impurities are removed.

Change in temperature: The reaction might get hotter (exothermic) or colder (endothermic). Think of the heat produced when burning wood for cooking, a common practice in some areas of South Africa.

Production of light: Burning a match or fireworks displaying light.

Examples of Chemical Changes: Burning wood (combustion): Wood reacts with oxygen in the air to produce ash, carbon dioxide, water, and heat. This is vital in many South African communities for cooking and heating.

Rusting of Iron: Iron reacts with oxygen and water to form iron oxide (rust). This is a major concern for infrastructure in coastal areas of South Africa where humidity and salinity are high.

Photosynthesis: Plants use sunlight, water, and carbon dioxide to produce glucose (sugar) and oxygen. This is the foundation of our food chain and a vital process in our agriculture and ecosystems.

Digestion of Food: Our bodies break down food molecules into simpler substances that our cells can use for energy. This process involves many chemical reactions. Think of how "pap" made from maize meal, a staple food, is broken down in our bodies.

Neutralisation of Acids and Bases: Mixing an acid (like lemon juice) with a base (like baking soda) will produce salt and water. Physical Change vs.

Chemical Change: | Feature | Physical Change | Chemical Change | |-----------------|----------------------------------------------------|-------------------------------------------------------| | New Substance | No new substance is formed. | A new substance is formed. | | Composition | The composition of the substance remains the same. | The composition of the substance changes. | | Reversibility | Often easily reversible. | Usually difficult to reverse. | | Examples | Melting ice, boiling water, dissolving sugar. | Burning wood, rusting iron, cooking an egg. | Rate of Reaction: The rate of reaction describes how quickly a chemical reaction occurs. A fast reaction happens quickly, while a slow reaction takes a long time. Factors Affecting the Rate of Reaction (Introduction): We'll explore these in more detail later, but for now, it's important to understand that the speed of a reaction can be influenced by things like: Temperature: Generally, increasing the temperature increases the rate of reaction.

Concentration: Increasing the concentration of reactants usually increases the rate of reaction.

Surface Area: For reactions involving solids, increasing the surface area (e.g., using powdered instead of a solid chunk) can increase the rate of reaction.

Catalysts: Substances that speed up a reaction without being used up themselves.

Word Equations: We can represent chemical reactions using word equations, which are a simple way to describe the reactants and products involved.

Example: Iron + Oxygen → Iron Oxide (Rust) This means iron reacts with oxygen to form iron oxide, also known as rust.

Example: Methane + Oxygen → Carbon Dioxide + Water This is the reaction that occurs when we burn methane gas. Methane reacts with Oxygen to form Carbon Dioxide and Water.

Example: Hydrogen Peroxide → Water + Oxygen Hydrogen peroxide decomposes to form water and oxygen. Guided Practice (With Solutions)

Question 1: Identify which of the following is a physical change and which is a chemical change: a)

Cutting hair b)

Burning paper Solution: a) Cutting hair is a physical change. Although the hair is shorter, it is still hair. No new substance is formed. b) Burning paper is a chemical change. The paper is converted into ash, smoke, and gases. New substances are formed.

Question 2: In the reaction of methane (CH4) with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O), identify the reactants and products.