I love it once teachers questioning questions! i alway gain a good science question due to the fact that they make me really think about the topic to be certain I provide the right answer, however when a teacher asks a question, i feel obligated to give solution (if i can) that will certainly let them explain the subject to your students. Recently, I have been talking with a teacher friend, about chemical and also physical changes, and also the more we talked, the more I believed the rest of you might find this useful.

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The principles of physical and chemical changes are contained in the elementary and also middle school curriculum for most states, and also students are expected to have the ability to tell even if it is a change is physics or chemical.

For a chemical change, at the very least one that the substances associated must wind up with a different chemical formula. For a physics change, a problem may readjust its physical properties (size, shape, color, etc.) but its chemistry formula remains the same.

Chemical change


Physical change

OK the sounds straightforward enough. If a chemistry formula changes, we have a chemical change. Because that example, if us mix vinegar and baking soda, the mixture bubbles and fizzes, and also we wind up with water, carbon dioxide, and sodium acetate.

CH3CO2H + NaHCO3 = H2O + CO2 + CH3COONa

The chemistry formulas at the end are various from the formulas at the start, i m sorry tells united state that this is a chemical change.

If the chemical formulas do not change, then any type of changes room physical changes. A an excellent example of that is dissolving sugar in water. As soon as the sugar has actually all dissolved, you have actually molecules that sugar combined in through the molecules of water. The chemical formulas at the end are the same as they to be at the start, so this is a physics change.

C12H22O11 + H2O = C12H22O11 + H2O

While that might sound favor a an easy distinction, unless you recognize the chemistry formulas because that the building material before and also after the change, you can’t be certain which sort of readjust it is. There are indications that a change MAY be a chemical change, such together a change of color, a change in temperature, providing off light, forming bubbles that gas, or developing a precipitate (solid particles), yet NONE of these space conclusive evidence of a chemical change. Any of lock can likewise be led to by a physical change.

Color change:

Mix part yellow food coloring v some blue food coloring and you obtain green, a physical readjust that results in a color change.

Change in temperature:

put some alcohol on your skin. Together it evaporates, her skin gets colder. This is a physical readjust (evaporation) that gives you a adjust in temperature. What around producing heat? The reusable warmth packs offered for muscle pain contain a at sight saturated solution of a chemical dubbed sodium acetate. As soon as the salt acetate crystallizes (a physical change), it offers off fairly a bit of heat, but the chemical formula remains the same.

Giving off light:

turn on a neon light. The neon gas is readjusted to plasma and also gives turn off light, however it is still neon, but it is now a plasma rather of a gas. This is a physical change.

Forming balloon of gas:

warmth a pan of water. Prior to it it s okay hot sufficient to boil, you will watch bubbles. Those bubbles room gases that were dissolved in the water. The is a physical readjust that produces bubbles of gas. Continue heating the water and it will certainly boil. Again, you get bubbles that gas, and also again the is a physics change.

Forming a precipitate:

A precipitate is a hard that creates in a solution. Dissolve street in water till no much more will dissolve. Let the water evaporate, and you acquire a precipitate (and rock candy!) This is a physical change.



many sources say that physical alters are reversible, when chemical alters are not. Allows test that through a common example that a physics change. Tear a paper of paper into pieces. No chemical formulas changed, therefore this is a physical change. Now reverse the adjust to placed the pieces earlier together right into a paper of paper. Walk ahead, and do it now. Okay wait.

Now allows look in ~ an even tougher example. If us melt part copper and also some zinc, and also then mix them together, we get a solution known as brass. The copper and zinc have actually not reacted chemically, so this is a physical change, but if you try to reverse the procedure to separate the copper and also zinc, you have actually a real challenge on your hands.

On the various other hand, numerous chemical alters are quickly reversible. You can use one electric current to rest water (H2O) into hydrogen (H2) and also oxygen (O2). The formulas have changed, so this is a chemistry change. One small spark will ignite the mixture the gases, developing water (H2O). You just reversed the readjust and wound up with the same substance you began with.

If friend have ever used litmus file or various other pH indicators, you have already seen an example of an conveniently reversible chemistry change. These indicators readjust their chemical formula depending upon the pH of their environment, and the chemistry reaction have the right to be reversed over and also over.


Another great example the reversible chemical alters is the carbon cycle. Respiration and photosynthesis are an excellent examples the chemical changes that space reversible. Cellular respiration switch oxygen come carbon dioxide. Photosynthesis counter carbon dioxide to oxygen. The very same molecules space chemically readjusted back and forth, over and also over and also over.

What it all boils down to is that unless you understand the chemistry formulas because that the building material before and after a change, there is no means to be sure if the change was chemical or physical. You have the right to make a guess, but until friend look at that chemical equation, you can’t be sure.

It gets also more challenging when friend look at some of the textbook examples of chemical and also physical changes. Because that example:

1. Is baking a cake an example of a chemical change or a physics change?

Stop here, and think around both instances carefully before you go any kind of further. No, ns really typical it. Stop. Think. Think some more. Then click here for the answer.


Obviously, the price is no. No? What sort of answer is that? Well, the inquiry was, “Is baking a cake an example of a chemical adjust or a physics change?” Actually, baking a cake is an instance of numerous chemical changes and also several physics changes, so that is no one OR the other. Instead, that is several instances of both kinds of change. Denaturing the proteins in the milk, eggs, and also flour are chemical changes. Chemistry bonds are broken, and new bonds are made. The salt bicarbonate and tartaric acid in the baking powder react chemically to develop carbon dioxide gas so her cake will certainly rise. That is likewise a chemistry change.

On the various other hand, if you taste the cake, the sugar has not adjusted chemically. That is still (C12H22O11), and it tho tastes favor sugar. It has actually been liquified in the fluid batter, however that is a physical change. If you put a piece of cake in a key of water, mash the up, strain it, and also evaporate the water, you gain the sugar back.

The water in the batter has actually not adjusted chemically. Quite a little bit of it has evaporated, however that is a physical change. If you included spices or flavorings to the cake, they most likely did not adjust chemically, yet some of their volatile chemicals were dissolved, and also diffused through the cake. If you melted the cake, then the taste will have readjusted too, and also that would be a chemical change. The alcohol and also water in her vanilla extract evaporated (physical change), leaving the vanillin unchanged to flavor her cake. When you look in ~ it, baking a cake contains several examples of both chemical and also physical changes.

OK, currently for the 2nd one. Again, think about it carefully before you go on. Then click here for the answer.

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Example #2 is part of a conversation ns am having at the moment. Dissolve salt in water is a generally used instance of a physical change, but when you dissolve salt (NaCl) in water (H2O), you get sodium ion (Na+) and also chlorine ion (Cl-) blended with the water (H2O). Some civilization claim the this is a chemical change, since the ionic link of sodium chloride has actually separated right into ions of sodium and also ions the chlorine. This conversation is quiet going on, so feel free to add your comments.

While this might seem really complex, when you start discovering chemistry, it becomes much easier. Together you study chemistry, you emphasis on chemical formulas and also equations. Looking in ~ a chemical equation will certainly instantly tell you whether the recipe have adjusted or not. Who would have thought the high school chemistry might be so much simpler than elementary institution science?