how do you know that the experience is a chemical change?


Adrian Dingle, Educator and Chemistry Author with 25 years teaching experience

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  • A chemical change involves the formation of a brand new substance.

Chemical changes are described by chemical equations where bonds are broken and made in order to form entirely new substances. At the AP level, we characterize the bonds that are being broken/made in chemical changes (reactions) as intra bonds (ionic and covalent).

Chemical changes often come with some observable change such as; production of a gas, a color change, production of a solid (a precipitate) or some kind of energy change (temperature goes up or down)

  • A physical change involves no new substances being formed, but rather the same substances being present but in a different form. The most common physical change would be a change of state from, for example, solid to liquid etc.

Physical changes can also be described by chemical equations, BUT here only the state symbols would change (there are no new substances formed). Here, only intermolecular forces are broken and made. At the AP level, we characterize the forces that are being broken/made in physical changes as intermolecular forces (H-bonds, London dispersion forces, dipole-dipole).

Brian Monetti, I took way too many science classes in college!

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This is a really awesome video! Here is another cool one to check out:

About the chemistry: Chemical changes occur when two or more substances combine to form a new substance. In this case, a handful of chemicals (hydrogen peroxide, sulfuric acid) are combined with some others (potassium iodine, sodium thiosulfate, and starch) Through a two step reaction, iodine is produced, which gives the blue color! Not too long ago, this deep purple iodine was used as a disinfectant, and would be used if you got a bad cut. Thankfully, Neosporin and other disinfectant/antibiotics have replaced the stinging and painful iodine!!

There are a few ways to tell if the change is either Chemical or Physical. A Chemical Change will require the combination of several substances to make a new one, which happens here (the creation of iodine from other chemicals.) On the other hand, Physical Changes only change a substance's form. An example of a Physical Change might be freezing of water into ice, or hammering a piece of metal into a flat sheet. In Physical Changes, new substances are not created, but only their forms are changed.

Hope this helps!!

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