Chemical Change Affected by Heat _ On grade_ 02

Finally, you hear the familiar sound of the timer on the oven ring. You rush to the kitchen, open the door of the oven a crack, and peek in. As you feel the heat pour out, you see the golden-brown color and smell the sweet aroma emanating from within. The large pan full of cookies looks absolutely incredible, and you can’t wait to try one! The color tells you they are perfectly done and ready to come out of the oven. You remove the pan from the oven, being careful not to burn yourself. Baking cookies is a fun and tasty way to spend a rainy day.

Earlier today, you carefully placed the blobs of raw cookie dough onto the greased pans. The gooey dough looks nothing like the cookies that just came out of the oven. What happens to the dough when it is in the oven, and why does it cause such a change? When the oven is turned on, it produces heat. It is the heat from the oven that causes the gooey blob of dough to transform into a yummy cookie.

Do you remember what you learned about physical changes? A physical change is one type of change that can occur to matter. In a physical change, the material’s size or shape may be affected, but the material itself stays the same. For example, a piece of paper can be folded, crumpled, or torn. In all cases, what is left is still paper.

A physical change can also include a change in state. For example, a solid ice cube melting is a change in state or phase change. Melting is a term used to describe what happens when solids turn into a liquid. Liquid water freezing is also a change in state. Freezing occurs when liquids, like water, change into a solid. These types of phase changes are easily reversible. This reversibility in phase changes makes them a physical change as it is proof the material is still made of the same substance.

Even with all the changes in state, the material is still the same material, no matter how many phase changes it may go through. An ice cube and water vapor are still forms of water. It has not changed into a something else, but that does not mean all changes of substances are physical changes. What is it called when a substance is changed into something else?