CH 104: Lesson 1a

 

Distinction between Chemical and Physical Changes

Now let's expand on the distinction between chemical reactions and physical changes that was alluded to earlier.

Chemical reactions involve changing the materials that you're dealing with into new materials, and the changes that you see are the result of the appearance of the new materials. The new material might have a different phase than the starting materials. For example, if you create something new from two liquids and it happens to be a gas, then you will get gas bubbles. There will be a new material with a new phase that wasn't there before. Also, you can observe chemical reactions by seeing color changes, if the new material has a different color than the starting materials. When a chemical reaction takes place, usually the temperature will also change. It will either go up or down, sometimes imperceptibly. Those are the kinds of changes that you might see. In chemical reactions the changes that you observe are caused by the creation of or loss of different kinds of materials.

Physical changes are similar in some ways because you are looking at the same kinds of things--changes in the phase, the color, the temperature. But the physical changes are changes in the condition of the material or changes caused by mixing materials together or taking them apart--just mixing or separating, not the creation of something new. For example, temperature changes are physical changes if they are caused by heating or cooling. If you add heat to something and the temperature goes up, that is a physical change. If you cool it off and the temperature goes down, that is a physical change. Phase changes are physical changes also, if they are caused by heating or cooling. If you take some water and you heat it up and it changes to gas, that is a physical change. You still have water, only now it is steam instead of liquid. Also included in physical changes are color changes that are caused simply by mixing or by diluting a material or, in some cases, even by heating. For example, if you turn on an electric range and the element gets hot, you can see a color change. That is also a physical change.

Physical changes and chemical changes have much in common in that you are looking at similar kinds of results. The difference is whether the appearances or differences that you see are from a change in the condition of the material or from a change in what materials are present. That's the kind of thing you need to look for to distinguish between a chemical reaction and a physical change. It takes an inference to make that distinction. Sometimes it is very difficult to make that distinction and in some cases even chemists can get into arguments sometimes over what constitutes a physical or a chemical change. For example, when salt dissolves in water, some people say that is simply a physical change because if you evaporate away the water, you have the salt back again. Other people argue that there is a chemical change because the material present in solution has different properties than the separated pure water and pure salt. The solution will conduct electricity and the pure water and pure salt will not. So, there are cases where the distinction between physical and chemical changes gets kind of blurred. In a few weeks we will have more criteria we can use to distinguish between physical and chemical changes.

 

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