Lesson Notes By Weeks and Term v5 - Grade 4

Lesson Video

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Performance objectives

• Describe what happens when materials are heated or cooled.
• Identify changes as reversible or irreversible.
• Explain the difference between these two types of changes.
• Predict if a change is reversible or irreversible.

Lesson summary

This week, we'll be diving deeper into the fascinating world of matter and materials! Matter is everything around us – the air we breathe, the water we drink, the desks we sit at, and even ourselves! Understanding the properties of different materials and how they can change is important for so many reasons. Think about building a house – you need to know which materials are strong enough to support the roof. Or think about cooking – you need to understand how heat affects different ingredients to make delicious food! In South Africa, we rely on a variety of materials for everything from building schools and hospitals to crafting traditional art and tools.

Lesson notes

What Happens When We Heat Materials? Heating a material adds energy to it. This energy makes the particles that make up the material move faster. What happens next depends on the material itself.

Solids: When you heat a solid, like chocolate, the particles vibrate faster and faster. Eventually, they have enough energy to overcome the forces holding them in place, and the solid melts into a liquid. Think about leaving a chocolate bar in the sun in South Africa – it becomes soft and melty!

Liquids: When you heat a liquid, like water, the particles move even faster. Some of them gain enough energy to escape the liquid and become a gas (steam). This is called evaporation. Think about boiling water for tea or cooking pap.

Gases: When you heat a gas, the particles move even faster and spread out more. What Happens When We Cool Materials? Cooling a material removes energy from it. This makes the particles move slower.

Gases: When you cool a gas, like steam, the particles slow down and come closer together. Eventually, they can turn back into a liquid. This is called condensation. Think about the water droplets that form on the outside of a cold glass of juice on a hot day.

Liquids: When you cool a liquid, like water, the particles slow down even more. They eventually lose enough energy to become a solid (ice). This is called freezing. Think about making ice blocks in the freezer.

Solids: When you cool a solid, the particles just vibrate slower, but it stays solid. Reversible vs.

Irreversible Changes Reversible Change: A reversible change is one where you can get back the original material. Heating ice to make water, then cooling the water back to ice is a reversible change. The water is still water, just in a different state. Think about freezing water into ice and melting it again. This is reversible. Another example is dissolving sugar in water. You can evaporate the water and get the sugar back.

Irreversible Change: An irreversible change is one where you cannot easily get back the original material. Burning wood is an irreversible change. Once the wood is burned to ash, you cannot turn the ash back into wood. Think about baking a cake. You can't turn the baked cake back into the separate ingredients (flour, eggs, sugar). Other examples include cooking an egg, rusting metal, or burning paper.

Examples: Heating Ice: Ice is a solid form of water. When you heat ice, it melts and turns into liquid water. When you cool the water again, it freezes back into ice. This is a reversible change.

Burning a Candle: When you burn a candle, the wax melts and burns, producing light and heat. The wax is changed into gases like carbon dioxide and water vapor. You cannot easily turn these gases back into wax. This is an irreversible change.

Mixing Sand and Water: When you mix sand and water, the sand settles to the bottom, but you can still separate them by pouring the water off. The sand is still sand, and the water is still water. This is a reversible change.

Cooking an Egg: When you cook an egg, the proteins in the egg change their structure, making the egg firm and solid. You can't uncook an egg. This is an irreversible change. Guided Practice (With Solutions)

Question 1: You leave a block of butter on the kitchen counter on a hot day in Durban. What happens to the butter? Is this a reversible or irreversible change?

Solution: What happens: The butter will melt. Butter is a solid at cooler temperatures, but it melts into a liquid when heated.

Reversible or Irreversible: This is a reversible change because you can put the melted butter in the refrigerator and it will solidify back into butter.

Question 2: You burn a piece of paper in a fire. What happens to the paper? Is this a reversible or irreversible change?

Solution: What happens: The paper burns and turns into ash, smoke, and gases.

Reversible or Irreversible: This is an irreversible change because you cannot turn the ash, smoke, and gases back into paper.

Question 3: You dissolve sugar in a glass of water to make cool drink. Can you get the sugar back? Is dissolving sugar a reversible or irreversible change? How could you get the sugar back?

Solution: What happens: The sugar disappears into the water, creating a sweet cool drink.

Reversible or Irreversible: This is a reversible change because you can get the sugar back by evaporating the water.

How to get the sugar back: If you were to leave the sugar water in the sun, the water would evaporate and the sugar would be left behind.

Question 4: A metal bicycle is left outside in the rain for a long time. It begins to rust. Is this change reversible or irreversible? Explain why.

Solution: What happens: The metal reacts with the oxygen and water in the air, forming rust (iron oxide).

Reversible or Irreversible: This is an irreversible change. While it's possible to remove rust, you can't easily turn the rust back into the original metal. The metal's composition has been altered.