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Learn how matter can change its look without changing what it is made of.
Have you ever wondered if an ice cube is still 'water' even when it's a hard, cold brick? What if you crumple a piece of paper—does it magically turn into a new material, or is it just having a bad hair day?
Imagine you put on a superhero costume. You look different, and you might even act different, but underneath the mask, you are still you. A physical change is exactly like that! It is a change in the way matter looks—like its shape, size, or state—without changing what the matter actually is. If you break a graham cracker, you have two pieces, but they are both still graham crackers. No new substance was created. We call the 'stuff' that things are made of matter, and in a physical change, the identity of that matter stays exactly the same.
1. Take one whole sheet of notebook paper. 2. Use your hands to tear the paper into 10 small scraps. 3. Look at the scraps. Even though they are smaller and have messy edges, they are still made of paper. 4. Because the 'what' (paper) didn't change, only the 'size' changed, this is a physical change.
Quick Check
If you use a saw to cut a wooden log into sawdust, is the sawdust still wood?
Answer
Yes, it is still wood! It just changed its size and shape.
One of the coolest physical changes happens when things get hot or cold. Water is a great example. Water can be a solid (ice), a liquid (the stuff you drink), or a gas (steam). When ice melts, it looks different and feels different, but it is still made of the same molecules. Most physical changes are reversible, which means you can often change them back. If you freeze the water, it becomes ice again! Whether it is solid, liquid, or gas, the 'recipe' for the water never changed.
1. A puddle of water sits on the sidewalk after a rainstorm. 2. The sun comes out and heats the water. 3. The liquid water turns into an invisible gas called water vapor through evaporation. 4. Even though you can't see the gas, the water atoms are still there, just floating in the air!
Quick Check
Is melting a chocolate bar a physical change?
Answer
Yes, because it is still chocolate, just in a liquid form instead of a solid bar.
Why do scientists say the matter doesn't change? It's because of atoms. Atoms are the tiny building blocks of everything. During a physical change, the atoms might move faster, spread apart, or get squished together, but they do not hook up in new ways to create a different substance. Think of a box of LEGO bricks. If you build a tower and then break it down to build a house, you still have the exact same bricks. You just moved them around! In a physical change, the 'bricks' of matter stay the same.
1. Take a glass of clear water and a spoonful of white salt. 2. Stir the salt into the water until you can't see the white crystals anymore. This is called dissolving. 3. It looks like the salt is gone, but if you taste the water, it's salty! 4. The salt atoms are just hiding between the water atoms. If you let the water evaporate, the salt crystals will be left behind at the bottom.
Which of these is the best definition of a physical change?
Which of the following is an example of a physical change?
During a physical change, the atoms of a substance change into a completely new type of matter.
Review Tomorrow
In 24 hours, try to name three physical changes you saw in your kitchen (like slicing bread or melting butter).
Practice Activity
Try the 'Saltwater Challenge': Mix salt into water until it disappears, then leave the glass in a sunny window for a few days to see if the salt reappears when the water evaporates!