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Exploring changes that are permanent and cannot be undone.
Have you ever wondered why you can melt an ice cube and freeze it back again, but you can never turn a piece of toast back into soft, white bread?
In science, some changes are like a one-way street. Once you go down that path, you cannot go back! We call these irreversible changes. When a material changes irreversibly, it turns into something completely new. Think about a piece of paper. If you fold it, you can unfold it. That is reversible. But if you burn that paper with a match, it turns into gray ash and smoke. You can't 'un-burn' the ash to get your paper back. The paper has been changed permanently.
Let's look at what happens in a frying pan: 1. Start with a raw egg. It is liquid, runny, and clear. 2. Add heat from the stove. 3. The egg becomes solid and white. 4. Even if you put the egg in the fridge to cool it down, it stays solid. It will never be a runny, raw egg again!
Quick Check
If a change is 'irreversible,' can we turn the material back to how it was at the start?
Answer
No, an irreversible change is permanent and cannot be undone.
Most of the time, adding lots of heat causes an irreversible change. When we bake a cake, we mix flour, sugar, and eggs. Once the heat of the oven hits that mixture, a chemical reaction happens. The liquid batter turns into a fluffy solid. You can't pull the eggs or flour out of a finished cake! Burning is another example. When wood burns, it reacts with oxygen () in the air to create ash and gases. The wood is gone forever, replaced by new materials.
Imagine you are making a cookie: 1. You mix soft butter, grainy sugar, and white flour. 2. You put the dough in a hot oven at . 3. The heat changes the smells, the colors, and the textures. 4. The result is a crunchy, brown cookie. You have created a new material that cannot be turned back into separate butter and flour.
Quick Check
What is one common thing we add to materials to cause an irreversible change?
Answer
Heat (or fire).
Sometimes, just mixing two things together can cause an irreversible change, even without heat! If you mix vinegar with baking soda, you will see lots of bubbles. Those bubbles are a gas called carbon dioxide (). Because a new gas was made and flew away into the air, you cannot easily put the vinegar and baking soda back the way they were. The materials have reacted to form something new!
This is a slow irreversible change: 1. A shiny iron nail is left outside in the rain. 2. The water and oxygen in the air touch the metal. 3. Over many days, a chemical change happens. A orange-brown flaky powder called rust forms on the surface. 4. The shiny iron has turned into rust. You cannot simply wash the rust off to turn it back into the iron that was lost.
Which of these is an example of an irreversible change?
What often happens to a material during an irreversible change?
Burning a piece of wood is a reversible change because you still have the ash left over.
Review Tomorrow
Tomorrow morning at breakfast, look for one irreversible change happening in the kitchen. Can you explain why it can't be undone?
Practice Activity
With an adult, find a piece of raw pasta and a piece of cooked pasta. List three ways the pasta changed after being heated in water.