I love it when teachers ask questions! i alway reap a great science question since they do me really think around the subject to be sure I offer the best answer, yet when a teacher asks a question, i feel obligated come give response (if ns can) that will certainly let them describe the topic to your students. Recently, I have actually been talking with a teacher friend, about chemical and physical changes, and the an ext we talked, the more I assumed the remainder of you might find this useful.

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The ideas of physical and chemical alters are included in the elementary and middle college curriculum for many states, and also students space expected to be able to tell whether a adjust is physics or chemical.

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

Chemical change


Physical change

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

CH3CO2H + NaHCO3 = H2O + CO2 + CH3COONa

The chemical formulas in ~ the end are various from the formulas at the start, i beg your pardon tells united state that this is a chemistry change.

If the chemical formulas perform not change, then any changes space physical changes. A good example of that is dissolving sugar in water. As soon as the sugar has all dissolved, you have actually molecules of sugar mixed in through the molecule of water. The chemistry formulas in ~ the finish are the same as they were at the start, therefore this is a physics change.

C12H22O11 + H2O = C12H22O11 + H2O

While that may sound prefer a basic distinction, uneven you know the chemical formulas because that the building materials before and also after the change, you can’t be certain which type of adjust it is. There are signs that a readjust MAY be a chemical change, such as a adjust of color, a readjust in temperature, offering off light, forming bubbles that gas, or developing a precipitate (solid particles), but NONE that these space conclusive evidence of a chemistry change. Any kind of of lock can additionally be caused by a physics change.

Color change:

Mix part yellow food coloring v some blue food coloring and also you get green, a physical readjust that outcomes in a shade change.

Change in temperature:

put some alcohol on her skin. As it evaporates, her skin gets colder. This is a physical change (evaporation) that provides you a readjust in temperature. What about producing heat? The reusable warmth packs offered for muscle pains contain a at sight saturated systems of a chemical referred to as sodium acetate. When the sodium acetate crystallizes (a physics change), it provides off quite a bit of heat, however the chemistry formula remains the same.

Giving off light:

rotate on a neon light. The neon gas is changed to plasma and gives off light, however it is quiet neon, yet it is currently a plasma rather of a gas. This is a physical change.

Forming balloon of gas:

warmth a pan the water. Before it it s okay hot sufficient to boil, you will see 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 also it will certainly boil. Again, you obtain bubbles of gas, and also again it is a physics change.

Forming a precipitate:

A precipitate is a hard that creates in a solution. Dissolve sugar in water until no much more will dissolve. Allow the water evaporate, and also you obtain a precipitate (and absent candy!) This is a physical change.



numerous sources say the physical changes are reversible, when chemical changes are not. Lets test that with a usual example that a physical change. Tear a sheet of file into pieces. No chemistry formulas changed, for this reason this is a physics change. Now reverse the adjust to placed the pieces back together right into a paper of paper. Go ahead, and also do that now. I’ll wait.

Now lets look in ~ an even tougher example. If us melt part copper and also some zinc, and then mix them together, we obtain a solution known as brass. The copper and zinc have not reacted chemically, for this reason this is a physical change, however if you try to reverse the process to separate the copper and zinc, you have actually a real difficulty on her hands.

On the various other hand, numerous chemical changes are conveniently reversible. You deserve to use an electric existing to rest water (H2O) right into hydrogen (H2) and oxygen (O2). The formulas have actually changed, for this reason this is a chemical change. One small spark will certainly ignite the mixture of gases, developing water (H2O). You simply reversed the readjust and wound up with the exact same substance you began with.

If you have ever used litmus file or various other pH indicators, girlfriend have already seen an instance of an conveniently reversible chemistry change. This indicators change their chemical formula depending on the pH of your environment, and the chemical reaction have the right to be reversed over and also over.


Another an excellent example that reversible chemical transforms is the carbon cycle. Respiration and also photosynthesis are great examples the chemical transforms that are reversible. Moving respiration switch oxygen come carbon dioxide. Photosynthesis switch carbon dioxide come oxygen. The exact same molecules are chemically adjusted back and forth, over and also over and over.

What it every boils down to is that uneven you know the chemical formulas for the building material before and after a change, over there is no way to be sure if the readjust was chemistry or physical. You can make a guess, but until you look at the chemical equation, you can not be sure.

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

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

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


Obviously, the answer is no. No? What type of price is that? Well, the inquiry was, “Is baking a cake an example of a chemical change or a physical change?” Actually, baking a cake is an instance of number of chemical changes and also several physical changes, so the is not one OR the other. Instead, it is several instances of both kinds of change. Denaturing the protein in the milk, eggs, and also flour room chemical changes. Chemical bonds space broken, and brand-new bonds room made. The salt bicarbonate and tartaric mountain in the baking powder reaction chemically to produce carbon dioxide gas so your cake will certainly rise. The is additionally a chemistry change.

On the other hand, if friend taste the cake, the sugar has not adjusted chemically. It is quiet (C12H22O11), and also it quiet tastes like sugar. It has actually been dissolved in the liquid batter, yet that is a physical change. If you put a piece of cake in a key of water, mash it up, strain it, and evaporate the water, you obtain the sugar back.

The water in the batter has actually not adjusted chemically. Quite a bit of it has evaporated, yet that is a physical change. If you added spices or flavorings come the cake, they probably did not change chemically, yet some of your volatile chemicals were dissolved, and diffused with the cake. If you melted the cake, climate the taste will certainly have adjusted too, and that would be a chemistry change. The alcohol and water in her vanilla extract evaporated (physical change), leave the vanillin unchanged to flavor her cake. Once you look in ~ it, baking a cake has several examples of both chemical and physical changes.

OK, currently for the second one. Again, think around it carefully before you go on. Then click right here for the answer.

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Example #2 is component of a conversation i am having at the moment. Dissolving salt in water is a generally used instance of a physics change, yet when girlfriend dissolve salt (NaCl) in water (H2O), you gain sodium ion (Na+) and also chlorine ions (Cl-) blended with the water (H2O). Some people claim the this is a chemistry change, due to the fact that the ionic link of sodium chloride has separated into ions that sodium and ions the chlorine. This conversation is still going on, for this reason feel free to include your comments.

While this might seem very complex, as soon as you start finding out chemistry, it becomes much easier. As you study chemistry, you emphasis on chemical formulas and equations. Looking in ~ a chemical equation will certainly instantly tell you even if it is the recipe have adjusted or not. Who would have actually thought the high school chemistry can be so much easier than elementary school science?