I love it as soon as teachers asking questions! ns alway gain a good science question due to the fact that they make me really think about the topic to be sure I offer the best answer, yet when a teacher asks a question, ns feel obligated come give response (if i can) that will let them describe the topic to your students. Recently, I have been talking through a teacher friend, around chemical and also physical changes, and the more we talked, the much more I assumed the rest of you could find this useful.

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The concepts of physical and also chemical changes are contained in the elementary and also middle institution curriculum for most states, and also students room expected to have the ability to tell whether a change is physical or chemical.

For a chemical change, at the very least one the the substances associated must wind up with a various chemical formula. Because that a physical change, a substance may adjust its physical properties (size, shape, color, etc.) but its chemical formula continues to be the same.

Chemical change


Physical change

OK the sounds an easy enough. If a chemistry formula changes, we have actually a chemical change. Because that example, if us mix vinegar and also baking soda, the mixture bubbles and fizzes, and we wind up through water, carbon dioxide, and 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 chemistry formulas perform not change, then any type of changes are physical changes. A good example of the is dissolving sugar in water. When the sugar has all dissolved, you have actually molecules that sugar blended in with the molecule of water. The chemistry formulas at the end are the exact same as they were at the start, so this is a physical change.

C12H22O11 + H2O = C12H22O11 + H2O

While that might sound prefer a an easy distinction, unless you recognize the chemical formulas because that the building materials before and after the change, you can’t be sure which type of adjust it is. There are indications that a adjust MAY it is in a chemistry change, such as a adjust of color, a change in temperature, providing off light, creating bubbles of gas, or forming a precipitate (solid particles), however NONE of these are conclusive evidence of a chemistry change. Any type of of lock can likewise be led to by a physics change.

Color change:

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

Change in temperature:

placed some alcohol on your skin. As it evaporates, your skin it s okay colder. This is a physical change (evaporation) that gives you a readjust in temperature. What around producing heat? The reusable warmth packs marketed for muscle ache contain a supervisor saturated equipment of a chemical referred to as sodium acetate. When the sodium acetate crystallizes (a physics change), it gives off quite a little of heat, however the chemical formula remains the same.

Giving turn off light:

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

Forming bubbles of gas:

warm a pan the water. Before it gets hot sufficient to boil, friend will check out bubbles. Those bubbles space gases the were liquified in the water. The is a physical readjust that produces bubbles of gas. Proceed heating the water and it will certainly boil. Again, you acquire bubbles that gas, and also again the is a physical change.

Forming a precipitate:

A precipitate is a solid that develops in a solution. Dissolve sugar in water until no much more will dissolve. Permit the water evaporate, and also you acquire a precipitate (and absent candy!) This is a physical change.



countless sources say the physical changes are reversible, while chemical changes are not. Allows test that through a typical example the a physics change. Tear a paper of document into pieces. No chemistry formulas changed, so this is a physical change. Currently reverse the change to placed the pieces back together right into a paper of paper. Walk ahead, and also do it now. Ok wait.

Now allows look in ~ an even tougher example. If us melt some copper and some zinc, and also then mix lock together, we acquire a solution known as brass. The copper and zinc have actually not reacted chemically, for this reason this is a physics change, however if you shot to turning back the process to separate the copper and also zinc, you have a real an obstacle on your hands.

On the other hand, countless chemical transforms are easily reversible. You have the right to use an electric current to break water (H2O) right into hydrogen (H2) and also oxygen (O2). The formulas have actually changed, for this reason this is a chemical change. One small spark will certainly ignite the mixture that gases, producing water (H2O). You simply reversed the readjust and wound up v the same substance you began with.

If girlfriend have ever used litmus document or various other pH indicators, you have currently seen an example of an easily reversible chemical change. This indicators readjust their chemistry formula relying on the pH of your environment, and also the chemistry reaction have the right to be reversed over and also over.


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

What it every boils under to is that uneven you know the chemistry formulas because that the building material before and also after a change, there is no way to be sure if the change 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 complicated when girlfriend look at several of the textbook instances of chemical and also physical changes. Because that example:

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

Stop here, and think about both examples carefully before you go any further. No, ns really median it. Stop. Think. Think some more. Then click below for the answer.


Obviously, the answer is no. No? What kind of price is that? Well, the concern was, “Is baking a cake an instance of a chemical adjust or a physics change?” Actually, baking a cake is an instance of number of chemical changes and also several physical changes, so it is no one OR the other. Instead, the is several instances of both type of change. Denaturing the proteins in the milk, eggs, and also flour room chemical changes. Chemical bonds space broken, and brand-new bonds are made. The sodium bicarbonate and also tartaric acid in the baking powder react chemically to create carbon dioxide gas so her cake will rise. The is additionally a chemical change.

On the other hand, if you taste the cake, the sugar has not readjusted chemically. The is still (C12H22O11), and it still tastes favor sugar. It has been liquified in the fluid batter, but that is a physical change. If you placed a item of cake in a bowl of water, mash the up, stress, overload it, and evaporate the water, you get the sugar back.

The water in the batter has actually not changed chemically. Quite a little of it has actually evaporated, but that is a physics change. If you included spices or flavorings to the cake, they most likely did not adjust chemically, however some of their volatile chemicals to be dissolved, and also diffused v the cake. If you melted the cake, climate the taste will have readjusted too, and that would certainly be a chemistry change. The alcohol and water in your vanilla extract evaporated (physical change), leaving the vanillin the same to flavor your cake. As soon as you look in ~ it, baking a cake consists of several instances of both chemical and physical changes.

OK, now for the 2nd one. Again, think about it carefully prior to you walk on. Then click here for the answer.

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

While this might seem an extremely complex, once you start learning chemistry, that becomes lot easier. Together you examine chemistry, you emphasis on chemical formulas and equations. Looking at a chemical equation will instantly tell you whether the recipe have adjusted or not. That would have actually thought the high college chemistry can be therefore much simpler than elementary college science?