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We may not often notice it but things around us change all the time. Our plants grow day by day; clouds form and fall as rain; the food we eat browns while it cooks in the pan. Change, as the saying goes, is the only thing that remains constant.
There are different kinds of change and, in this lesson, we will be talking about physical change and chemical change.
What is physical change and what is chemical change? How are they different?
A physical change is just a change in the physical property of an object – its size, shape, texture, or state of matter. No new substance is created.
The most common example of a physical change is what happens to water. When water freezes, melts, evaporates, or condenses, it only undergoes physical change. It may change its state from liquid to solid (when it freezes), solid to liquid (melts), liquid to gas (when it evaporates), or gas to liquid (when it condenses), but whether it is solid, liquid, or gas, it is still water. It doesn’t become another substance. For that reason, it is only a physical change.
Not all physical changes are reversible – for example, if you cut a pork chop into tiny pieces, you’re not gonna be able to turn them all back into a large pork chop. However, if a change is reversible, that’s a pretty good clue that the change is only a physical change.
Again, take the example of water. When water freezes, you can melt the ice and it will turn back to liquid.
When you put butter in a warm pan, it melts, but you can turn it back to a solid by putting it in the fridge.
You can fold a paper and then unfold it.
You can bend a piece of wire and straighten it back again.
These are all examples of changes that can be reversed and that gives you a pretty good clue that they are physical, not chemical, changes.
Properties of matter:
Now, you can fold a piece of paper, and it will still be paper, but if you burn it, when the flames die down, what you’ll have left is no longer paper – just ash. This is an example of a chemical change.
A chemical change is a change in what something is – a change in what something is made of. An entirely different substance is created when a chemical change occurs and this change generally cannot be reversed.
The paper, when burned, becomes ash. The ash won’t turn back into paper even if you try to freeze or put water on it.
Fruit and vegetable scraps can decompose in soil and turn into compost, which cannot be turned back into peels and soil. (We discuss more chemical changes of this sort in Materials That Undergo Decay.)
Flour, eggs, sugar, and butter, when baked, become cake. If you take it out of the oven and try to cool it, it will simply become, well, a cooled cake. It won’t revert back to its ingredients.
There are signs that can give you a clue that you’re dealing with a chemical change. They don’t always mean that there’s a chemical change going on but their presence is a pretty good clue, especially when more than one of them is present.
Examples of physical changes
Examples of chemical changes
Now that we’ve talked about the kinds of changes that can happen to the things around us, let’s talk about how these changes are brought about.
You’ll notice that a lot of the examples that we talked about earlier have one thing in common: they involve heat.
Whether it’s water evaporating due to the heat in the surroundings, or paper burning due to the heat from the fire, or food cooking due to the heat from the stove or the oven, heat is responsible for a lot of changes.
Changes brought about by heat can be physical or chemical.
Heat can help you change the shape of metal or melt a block of butter. The metal will still be metal and the butter will still be butter so those are simply physical changes.
Heat is involved when wood is burned or food is cooked, and since new substances are formed (ash, scrambled eggs) from these processes, they are considered chemical changes.
Can you think of other physical or chemical changes caused by heat?
Another thing that plays a big role in a lot of the changes going on around us is oxygen.
I don’t know if you’ve noticed but when you slice eggplants or apples and you leave them exposed to the air for a while, they turn brown. It’s a complicated process but basically it starts when the oxygen in the air acts on an enzyme in the fruit called polyphenol oxidase. Some more stuff happens, and at the end of it, melanin – the same thing in our skin that makes it brown – is produced in the apple or eggplant. In short, it’s the oxygen in the air that starts the process that makes the melanin that turns apples and eggplants brown. You can see a time-lapse video of it here.) This process is called enzymatic browning (or enzymic browning) and it happens in lots of kinds of food: fruits and vegetables (such as apples, eggplants, bananas, avocados, and potatoes), coffee, cocoa beans, and even shrimp, just to name a few.
Another process that involves oxygen is combustion. This is when oxygen reacts with a fuel, such as wood, coal, diesel, gasoline, or liquefied petroleum gas (LPG), and creates heat. This can produce a fire (such as in a stove) or a hot gas (such as in a car). When we eat cooked food or ride a vehicle to school, we are actually enjoying the benefit of a chemical change produced by oxygen.
A third change that we’re always seeing around us that is brought about by oxygen is rusting. This is what happens when iron (or its alloys, such as steel) reacts with the oxygen in the air in the catalyzing (speeding up) presence of moisture. This reaction creates a new substance: iron oxides and hydroxides, or, as we simply call it, rust.
As you can probably tell, all of the changes involved here are chemical changes. There’s a color change or temperature change and there’s an entirely different substance created.
It’s very important to note that these things would not happen without the presence of oxygen. This is why…
Have you ever noticed a really bad smell in the air and then somebody says, “Ugh, somebody’s burning rubber”? If you haven’t, you’re lucky. Burning rubber makes a really bad smell. The heat changes the rubber into several different substances and some of those new substances are gases that smell really awful.
Plus – not only do they smell horrible, the gases released from burning rubber pollute the air, making it unhealthy for us to breathe.
This is just one of the ways that changes caused by heat and oxygen affect the environment.
Changes in materials don’t only affect the material. They can also affect the environment the material is in. We should remember this so we can make better choices about what we do with our things.
For example, you have an old rubber tire. Do you:
On a bigger scale, burning coal releases heat-trapping gases (called greenhouse gases) into the air, which prevents the earth from transferring heat back to space. The heat is trapped here and now our planet is becoming much warmer. Unfortunately, coal is burned all over the world to produce electricity – and we all use electricity. So what can we do? Do we…
It’s not only chemical changes that can have harmful effects on the environment. Cutting down a tree is just a physical change – it doesn’t produce harmful new substances. BUT when you cut down a lot of trees, the surrounding area gets warmer. The trees could have absorbed carbon dioxide (a greenhouse gas) and given off oxygen but now they can’t. Their roots could have absorbed rainwater but now, when there’s a lot of rain, the water just slides off the ground and causes floods. The roots could also have held on to the soil but now the soil just gets eroded and carried down to the sea, where it can destroy the fish’s food and smother the places where fish lay eggs. So, you see, even a simple physical change can have a disastrous effect on the environment.
This is a lot to think about and of course you can’t solve all the world’s problems at once. But by being aware of the changes happening around you, you can make better choices. When more people make better choices, that’s when good change happens!
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