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Learn the difference between changing how something looks and changing what something is.
If you tear a piece of paper, it's still paper. But if you burn it, it turns into ash and smoke. Why can you tape the torn paper back together, but you can't 'un-burn' the ash?
A physical change happens when the appearance of an object changes, but the substance itself stays the same. Think of it like changing your clothes; you look different, but you are still the same person! Common physical changes include changes in state (like ice melting into liquid water) or changes in shape (like crushing a can). In a physical change, the molecules, such as , remain exactly the same before and after the change. Most physical changes are reversible, meaning you can often undo them.
1. Take a solid ice cube ( in solid form). 2. Add heat until it melts into liquid water ( in liquid form). 3. Notice that even though it looks different, it is still water. You can freeze it back into an ice cube easily!
Quick Check
If you break a glass window into tiny pieces, is it a physical or chemical change?
Answer
It is a physical change because the pieces are still made of glass; only the shape and size changed.
A chemical change occurs when substances react to form a completely new substance. This is like baking a cake; once the flour, eggs, and sugar turn into a fluffy cake, you can't turn them back! During a chemical change, atoms rearrange themselves to create new molecules. For example, when iron () reacts with oxygen (), it creates a new orange substance called rust (). Chemical changes are usually irreversible—you cannot easily get the original ingredients back.
1. Leave a shiny iron bicycle outside in the rain. 2. Oxygen from the air and water react with the iron. 3. A new, crumbly orange substance (rust) forms on the surface. This is a chemical change because the iron has turned into iron oxide ().
Quick Check
What is the main difference between a physical change and a chemical change?
Answer
A chemical change creates a brand new substance, while a physical change only changes the appearance or state.
Since we can't see atoms moving, how do we know a chemical change happened? Scientists look for clues! These include: 1. Color Change (like a leaf turning brown), 2. Gas Production (seeing bubbles or smelling an odor), 3. Temperature Change (the reaction gets hot or cold on its own), and 4. Light Production (like a firework). If you see these signs, a chemical reaction is likely taking place!
1. Mix liquid vinegar with solid baking soda. 2. Observe the immediate fizzing and bubbling. 3. The bubbles are Carbon Dioxide () gas being created. Because a new gas was formed that wasn't there before, this is a chemical change!
Which of the following is a physical change?
If you see bubbles forming when you mix two liquids, what is likely happening?
Tearing a piece of paper is a chemical change because you cannot perfectly put it back together.
Review Tomorrow
In 24 hours, try to list the four 'clues' that tell you a chemical change is happening.
Practice Activity
Go into your kitchen and find one example of a physical change (like cutting fruit) and one example of a chemical change (like frying an egg).