Lesson Notes By Weeks and Term v5 - Grade 6

Lesson Video

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Performance objectives

• Define and give examples of reversible and irreversible changes.
• Tell the difference between them.
• Predict if a change is reversible or irreversible.
• Describe materials before and after a change.
• Investigate these changes safely.

Lesson summary

This week, we're diving into the fascinating world of matter and materials, specifically looking at how they can change. We'll explore reversible and irreversible changes. Understanding these changes is crucial because it helps us understand the world around us, from cooking pap (maize porridge) in the kitchen to understanding why we can't un-burn firewood. Knowing the difference between reversible and irreversible changes allows us to make informed decisions about the materials we use every day and appreciate the chemical and physical processes happening constantly around us. Think about recycling, cooking, building, and even the weather!

Lesson notes

What are Changes in Matter? Matter is anything that has mass and takes up space. Think of the desk you're sitting at, the air you breathe, or the water you drink – all of these are examples of matter. Matter can undergo changes, and these changes can be either reversible or irreversible.

Reversible Changes: A reversible change is a change in a substance where the original substance can be recovered. This usually involves a change in state (solid, liquid, gas) or a change in physical appearance but not a change in the chemical composition of the substance. Key indicators of reversible changes are that no new substances are formed.

Examples of Reversible Changes: Melting and Freezing of Water: When ice (solid water) melts into liquid water, it's a reversible change. We can simply cool the liquid water back down to freeze it back into ice. The water molecules (H₂O) remain the same throughout the process. Why is this reversible? Because we are only changing the state of the water, not its chemical composition.

Dissolving Sugar in Water: When you stir sugar into water, it seems like the sugar disappears.

However, it's still there, just spread out (dissolved) amongst the water molecules. You can recover the sugar by evaporating the water (for example, by gently heating the sugar solution). As the water evaporates, the sugar crystals will reappear. Why is this reversible?* Because the sugar molecules are still sugar molecules (C₁₂H₂₂O₁₁) after being dissolved.

Folding a Piece of Paper: If you fold a piece of paper, you change its shape, but it is still paper. You can unfold it and return it to its original shape. Why is this reversible?* Because the chemical composition (cellulose) and basic properties of the paper haven't changed.

Irreversible Changes: An irreversible change is a change where the original substance cannot be easily recovered, and a new substance is formed. This usually involves a chemical reaction, where the molecules of the original substance are rearranged to form new molecules. Key indicators are the formation of new substances with different properties.

Examples of Irreversible Changes: Burning Wood: When you burn wood, it changes into ash, smoke, and gases. You cannot easily turn the ash, smoke, and gases back into wood. Burning is a chemical reaction called combustion, where the wood reacts with oxygen in the air to form new substances like carbon dioxide and water vapor. Why is this irreversible?* Because the wood's molecules (primarily cellulose) are broken down and rearranged into completely different molecules.

Cooking an Egg: When you cook an egg, the liquid egg white and yolk solidify and change in texture and appearance. You cannot easily turn the cooked egg back into its original liquid state. The heat causes the proteins in the egg to unfold and link together in a process called denaturation, forming a new structure. Why is this irreversible?* Because the proteins in the egg have undergone a chemical change and can't easily be reversed to their original state.

Rusting of Iron: When iron reacts with oxygen and water, it forms rust (iron oxide). Rust is a different substance from iron, with different properties. You can't simply turn rust back into iron. Why is this irreversible?* Because the iron atoms have combined with oxygen atoms to form a new compound with different properties. Think about the tools our grandparents use; they are susceptible to rust, and once rusted, they rarely regain their original strength and shine without significant industrial processing.

Baking Bread: The ingredients (flour, yeast, water) undergo chemical reactions during baking. The yeast produces carbon dioxide, which makes the bread rise. Once baked, you can't easily return the bread to its original ingredients. Why is this irreversible?* The starches and proteins in the flour have been chemically altered by the heat and the action of the yeast.

Key Differences Summarized: | Feature | Reversible Change | Irreversible Change | |-------------------|---------------------------------------------------|--------------------------------------------------| | New Substances | No new substances are formed. | New substances are formed. | | Recovery | Original substance can be recovered easily. | Original substance cannot be easily recovered. | | Example | Melting ice, dissolving salt in water | Burning wood, cooking an egg | Guided Practice (With Solutions)

Question 1: Imagine you have a candle. You light it, and it burns for a while. Is this a reversible or irreversible change? Explain your reasoning.

Solution: This is an irreversible change. When the candle burns, the wax reacts with oxygen in the air to produce carbon dioxide, water vapor, and light and heat. These are new substances. You cannot easily turn the carbon dioxide, water vapor, and remaining wax back into the original unburnt candle wax.

Commentary: This question tests the understanding of forming new substances.