Breathing, cooking, exercising, and more: we are surrounded by changes every day! But how do we keep straight which are physical and which are chemical?
Physical changes involve chemicals getting rearranged in a new way, but without getting destroyed or irreversibly combined. The molecular structure of the original substance(s) is the the same after the change, which means it is still the same substance. A common example is of water freezing; the liquid water becomes a solid, but it is still water. Ripping paper is another example because while it may be in pieces, it’s still paper. Signs of a physical change include:
- Expected color change
- Change in size or shape
- Change in state of matter
- No new substance formed!
Chemical changes, or reactions, involve chemicals getting rearranged in irreversible ways. We bring in the term “reaction” when dealing with chemical changes because the chemicals involved actually react with one another to form a new substance (that’s why we call the parts in a chemical reaction the reactants). The ways chemicals can react are many: two substances can combine to create a totally new one (Direct Combination), a substance can permanently break apart, separating into different substances (Decomposition), or maybe a substance combusts as it reacts to oxygen (Combustion) (see more types of reactions here). Whatever the reaction, it’s important to recognize that one or more new substances are formed! Burning wood is a common example of a chemical reaction because as the wood reacts with oxygen after it has been ignited, it creates smoke and ashes. Signs of a chemical reaction include:
- Unexpected color change
- Change in temperature as energy is released or absorbed
- Gas created
- New substance formed!
In many cases, it may seem a simple task to distinguish between the two, but it gets more complex when we’re dealing with changes that seem irreversible when they’re not. For example, we may look at a glass of Kool-Aid and think to ourselves, “I made Kool-Aid. It was water and a packet, but now it’s dissolved together to make new drink, so it’s a new substance.” However, if you think back to the list of signs of a physical change, you’ll realize that not only was there an expected color change (the water didn’t turn purple when you added red Kool-Aid!), but that it is reversible–the water could evaporate in the form of gas, leaving red Kool-Aid residue behind! The water is still water, and the Kool-Aid is still Kool-Aid, and since no new substances were formed, it is a physical change!
Another example of confusion is in cooking eggs–after all, isn’t it still an egg before and after tossing it in pan? However, on a molecular level, the egg has changed completely as the proteins have bonded in new ways, making it a new substance. Additionally, because of the change, it is not possible for the egg to ever become raw again, and since a new substance was formed, it is a chemical change!
For some practice using the different lists of signs for each type of change, and to view several other examples in action, check out this Prezi below!
Photo credit: Faris Algosaibi
2 Replies to “Chemical vs. Physical Changes: Let’s Get it Straight!”
Why do you include reversibility and irreversibility as a way to distinguish between physical and chemical changes? Most chemical changes are in fact reversible, and many physical changes are irreversible. For example, cracking an egg cannot be reversed. Cutting or tearing something is irreversible. Chopping wood is irreversible but it is a physical change. This is a bit of a pet peeve of mine as I have to unteach it in high school.
I included it because it can be a helpful way for some students to think about it since the vast majority of chemical changes are irreversible (and those that are require further chemical reactions). However, I did point out in my Prezi and to my 5th graders that there are many physical changes that cannot be reversed.