I love it as soon as teachers asking questions! i alway gain a good science question since they make me really think about the topic to be sure I offer the appropriate answer, but when a teacher asks a question, ns feel obligated to give an answer (if i can) that will let them explain the subject to your students. Recently, I have actually been talking with a teacher friend, around chemical and physical changes, and also the an ext we talked, the an ext I assumed the rest of you can find this useful.

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The concepts of physical and chemical transforms are consisted of in the elementary and also middle school curriculum for many states, and also students room expected to have the ability to tell even if it is a readjust is physics or chemical.

For a chemistry change, at least one of the substances associated must wind up with a different chemical formula. For a physical change, a problem may change its physical properties (size, shape, color, etc.) yet its chemistry formula continues to be the same.

Chemical change


Physical change

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

CH3CO2H + NaHCO3 = H2O + CO2 + CH3COONa

The chemistry formulas in ~ 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 carry out not change, then any type of changes room physical changes. A an excellent example of the is dissolving street in water. Once the sugar has actually all dissolved, you have molecules that sugar blended in v the molecule of water. The chemistry formulas in ~ the finish are the exact same as they to be at the start, for this reason this is a physical change.

C12H22O11 + H2O = C12H22O11 + H2O

While that may sound like a basic distinction, unless you know the chemistry formulas for the substances before and also after the change, you can’t be sure which kind of readjust it is. There are signs that a change MAY it is in a chemical change, such as a change of color, a adjust in temperature, offering off light, creating bubbles of gas, or developing a precipitate (solid particles), however NONE that these room conclusive proof of a chemistry change. Any of them can likewise be brought about by a physics change.

Color change:

Mix some yellow food coloring v some blue food coloring and also you obtain green, a physical change that outcomes in a color change.

Change in temperature:

placed some alcohol on her skin. Together it evaporates, your skin gets colder. This is a physical readjust (evaporation) that provides you a change in temperature. What around producing heat? The reusable warm packs marketed for muscle pains contain a supervisor saturated systems of a chemical dubbed sodium acetate. As soon as the sodium acetate crystallizes (a physics change), it provides off rather a little of heat, however the chemical formula stays the same.

Giving turn off light:

revolve on a neon light. The neon gas is adjusted to plasma and gives turn off light, yet it is tho neon, however it is currently a plasma instead of a gas. This is a physics change.

Forming balloon of gas:

heat a pan the water. Before it gets hot sufficient to boil, friend will view bubbles. Those bubbles are gases that were dissolved in the water. The is a physical adjust that produces balloon of gas. Continue heating the water and also it will certainly boil. Again, you acquire bubbles that gas, and again it is a physical change.

Forming a precipitate:

A precipitate is a heavy that develops in a solution. Dissolve sugar in water till no more will dissolve. Let the water evaporate, and you acquire a precipitate (and absent candy!) This is a physical change.



many sources say the physical transforms are reversible, when chemical alters are not. Lets test that through a usual example that a physical change. Tear a sheet of paper into pieces. No chemistry formulas changed, so this is a physics change. Currently reverse the readjust to put the pieces back together into a sheet of paper. Go ahead, and do that now. I’ll wait.

Now allows look at an also tougher example. If we melt part copper and some zinc, and also then mix castle together, we get a solution well-known as brass. The copper and also zinc have not reaction chemically, so this is a physics change, however if you shot to turning back the procedure to different the copper and zinc, you have actually a real an obstacle on her hands.

On the other hand, plenty of chemical changes are conveniently reversible. You have the right to use an electric existing to break water (H2O) into hydrogen (H2) and oxygen (O2). The formulas have changed, so this is a chemical change. One tiny spark will certainly ignite the mixture the gases, developing water (H2O). You just reversed the readjust and wound up through the same substance you began with.

If friend have ever used litmus document or other pH indicators, girlfriend have currently seen an instance of an conveniently reversible chemistry change. This indicators readjust their chemical formula depending upon the pH of your environment, and also the chemistry reaction deserve to be reversed over and over.


Another great example of reversible chemical changes is the carbon cycle. Respiration and photosynthesis are great examples that chemical transforms that room reversible. Cellular respiration converts oxygen to carbon dioxide. Photosynthesis converts carbon dioxide come oxygen. The same molecules space chemically readjusted back and forth, over and also over and also over.

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

It gets even more an overwhelming when girlfriend look at several of the textbook instances of chemical and also physical changes. For example:

1. Is baking a cake an instance of a chemical readjust or a physical change?

Stop here, and think around both examples carefully before you go any kind of further. No, i really mean it. Stop. Think. Think part more. Climate click right here for the answer.


Obviously, the answer is no. No? What sort of prize is that? Well, the concern was, “Is baking a cake an example of a chemical adjust or a physical change?” Actually, baking a cake is an example of numerous chemical changes and several physics changes, so the is not one OR the other. Instead, it is several examples of both type of change. Denaturing the proteins in the milk, eggs, and flour space chemical changes. Chemical bonds space broken, and new bonds are made. The salt bicarbonate and also tartaric mountain in the baking powder reaction chemically to develop carbon dioxide gas so her cake will rise. That is likewise a chemistry change.

On the various other hand, if girlfriend taste the cake, the sugar has actually not readjusted chemically. It is tho (C12H22O11), and it quiet tastes favor sugar. It has actually been liquified in the liquid batter, however that is a physics change. If you placed a piece of cake in a bowl of water, mash it up, stress, overload it, and also evaporate the water, you acquire the street back.

The water in the batter has not changed chemically. Rather a little bit of it has evaporated, however that is a physical change. If you included spices or flavorings to the cake, they probably did not adjust chemically, however some of their volatile chemicals were dissolved, and also diffused with the cake. If you burned the cake, climate the taste will certainly have changed too, and that would certainly be a chemistry change. The alcohol and water in her vanilla extract evaporated (physical change), leaving the vanillin the same to flavor your cake. As soon as you look at it, baking a cake includes several examples of both chemical and also physical changes.

OK, now for the second one. Again, think about it carefully before you go on. Climate click here for the answer.

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Example #2 is part of a conversation i am having at the moment. Dissolve salt in water is a frequently used example of a physical change, however when girlfriend dissolve salt (NaCl) in water (H2O), you gain sodium ion (Na+) and also chlorine ion (Cl-) combined with the water (H2O). Some people claim the this is a chemical change, due to the fact that the ionic link of sodium chloride has separated right into ions the sodium and also ions of chlorine. This conversation is still going on, therefore feel complimentary to include your comments.

While this may seem very complex, when you start discovering chemistry, the becomes much easier. Together you research chemistry, you focus on chemistry formulas and also equations. Looking in ~ a chemistry equation will instantly tell you whether the formulas have adjusted or not. That would have thought that high institution chemistry could be so much simpler than elementary institution science?