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A final look at everything learned about mixing, changing, and materials.
Have you ever wondered why you can melt an ice cube back into water, but you can't turn a baked cake back into flour and eggs?
Some changes are like a two-way street. These are called reversible changes. This means you can change a material and then turn it back to exactly how it was before. A great example is water. If you take liquid and put it in the freezer, it becomes solid ice. If you take it out, it melts back into liquid water! Nothing new was created; the water just changed its shape and feel. Other reversible changes include dissolving sugar in water or stretching a rubber band. In these cases, the physical properties change, but the material itself stays the same deep down.
1. Take a solid bar of chocolate. 2. Add heat by placing it in a warm bowl. 3. Watch it turn into a gooey liquid. 4. Place the liquid chocolate in the fridge. 5. It becomes a solid bar again! This is a reversible change.
Quick Check
If you freeze juice into a popsicle, is that a reversible or irreversible change?
Answer
It is a reversible change because you can melt the popsicle back into liquid juice.
Some changes are a one-way street. These are irreversible changes. Once the change happens, you can never go back to the original materials. This usually happens during a chemical reaction. When you bake a cake, the heat causes the flour, eggs, and sugar to bond together to make something entirely new. You can't 'un-bake' a cake to get the raw eggs back! Other examples include burning wood into ash or rusting metal. If you see smoke, smell something new, or see a permanent color change, it is likely an irreversible reaction.
1. Find a shiny iron nail. 2. Leave it outside in the rain and air. 3. The oxygen and water react with the iron. 4. A new orange substance called rust forms. 5. You cannot turn the orange rust back into shiny iron. This is irreversible.
Quick Check
What is one big sign that an irreversible chemical reaction has happened?
Answer
Signs include seeing smoke, a permanent color change, or something entirely new being created (like ash or gas bubbles).
It is important to know if you have a mixture or a reaction. In a mixture, different materials are put together, but they don't change into anything new. Think of a bowl of cereal. The milk and the cereal are together, but the cereal is still cereal! You could even pick the pieces out if you had enough time. In a reaction, the materials 'shake hands' and transform into something new. When you mix vinegar and baking soda, they create gas bubbles. The vinegar and soda are gone, and a new gas is made!
1. Pour vinegar into a plastic bottle. 2. Put baking soda inside a balloon. 3. Stretch the balloon over the bottle top and tip the soda in. 4. The materials react to create gas bubbles (). 5. The gas fills the balloon! This is a reaction because a new gas was created that wasn't there before.
Which of these is a reversible change?
If you mix sand and water in a bucket, what have you made?
When bubbles form after mixing two liquids, it is often a sign of a chemical reaction.
Review Tomorrow
Tomorrow, look around your kitchen. Can you find one thing that is a mixture and one thing that has gone through an irreversible change?
Practice Activity
Try this: Mix salt into a cup of water (a mixture). Ask a grown-up if you can leave it on a windowsill for a few days. When the water evaporates, will the salt still be there? (Spoiler: Yes, because it's reversible!)