A physical reaction is one where a change of state occurs. We went through most of this in the last lesson. Physical reactions do not change the substance itself, but may change its properties. A melting ice cube is a physical change. A chemical change involves creating a new substance. This is called a compound. These come in many forms, but always obey the rules of a chemical reaction.

Every compound contains two or more elements in a certain proportion. Table salt is a compound. Sodium and chlorine came together to create something completely new. As you may guess, there are many more compounds than there are elements. If you want to use the comparison of the elements being like letters, compounds are the words. Those letters can be range in an infinite number of ways. We can even add letters to existing words. In a similar fashion, elements can arrange themselves in an infinite number of ways. Elements or compounds are even added on to already existing compounds.

This new substance, the compound, has its own set of properties. Frying an egg is a chemical change. The properties of the cooked egg are very different than the properties of the raw egg. Another thing you might want to keep in mind is that usually a chemical reaction is not just a one-shot deal. Usually it is part of a chain reaction, like a multi-layered waterfall. For you to be able to move your arm, hundreds of chemical reactions are taking place inside your body.

There are several easy ways to tell if a chemical reaction has happened. The first one is if solid forms that wasn't there before. There's a very interesting example of this. If you take some water from this town in Pennsylvania and mix it with some water from this town in South Carolina, you would expect to just have water mixed together, right? Instead, a gel-like solid forms, which is actually burnable! Burning water! It's an amazing display. What is really going on is that there are dissolved ions in the water of both places. These ions get together and form what is called Sterno. That's the stuff you can buy in little cans to keep your food warm at a banquet.

Another obvious sign that a chemical reaction has taken place is that a gas is formed. If you drop a rusty nail into a cup of vinegar, you will see little bubbles appear and float to the top. Those bubbles are hydrogen gas being formed. When wine or beer is being made, gases are produced. That is one of the signs that the fermentation process has started. We all know what happens when we digest certain foods! That's a chemical change, too.

A color change will also show a chemical process. When the Statue of Liberty was first assembled, it was clear to see the shiny copper. It quickly changed to the green color we're familiar with now. That change was a chemical reaction between the copper and the oxygen in the atmosphere. That green copper oxide that developed is very differently than copper itself, and in fact protects the statue from further damage by the air. It creates a shell that stops more copper from reacting.

The last clear sign of a chemical reaction is that energy is either absorbed or given off. There's always some sort of an energy change. This energy can come in the form of heat, light, sound or electricity. When a matchstick burns, light and heat are given off. A photograph is made when the chemicals on the film absorb light. Fireworks give off both light and sound when they react. These are obviously chemical reactions. Can you think of any others?

Sometimes, more than one type of reaction is going on at the same time. You can have a physical reaction and a chemical reaction simultaneously. Remember the cooked egg? The protein in the egg is breaking down (a chemical process), but the water in it is just evaporating (a physical process).