You Know the Old Volcano Trick
So just what causes a chemical reaction anyway? You probably know that when you want to make a school science fair project you can make a “volcano" by combining baking soda and vinegar. However, even though you know that the combination of vinegar and baking soda makes the “eruption" happen in your “volcano," you may not fully understand that this is a classic example of a chemical reaction.
Molecules Are Always Moving
You can’t see it, but pretty much everything on earth has movement inside of it. The molecules that make up any substance are constantly moving. That movement is an important part of what causes chemical reactions.
Looking at any substance with the naked eye or even a regular microscope, you cannot see that molecular movement. Most often you need a powerful electron microscope, but most schools don't own one because they cost tens of thousands of dollars.
When Two Molecules Collide
Okay, so we know that chemical reactions happen when two substances come together. Technically, a chemical reaction happens with pretty much any two substances coming together. For example, if you took a bottle of water, added a bit of food coloring to it and waited, the food coloring would “react" with the water and eventually cause the water to become colored.
This is a classic chemical reaction as well, but not the one we usually associate with the term chemical reaction. The difference is that the reaction wasn’t violent, like our erupting volcano. Now, before we get to what causes the kind of violent chemical reaction you’re likely more interested in knowing about, let’s clear one thing up: not everything reacts chemically.
Why Chemical Reactions Might Not Occur
Take a cup, add some water and then slowly pour some oil into the same cup. The oil will rise to the top and fail to mix with the oil. In fact, if you got yourself a wick and floater (a piece of cork to hold the wick above the oil), you could stick it into the oil and light it, creating a kind of candle. The candle would keep burning until your oil was burned away and the water below it was left. The reason why this works is because oil and water don’t mix. That is, they don’t have a chemical reaction.
The reason that a chemical reaction doesn't happen is that the molecules weren’t able to line up with each other correctly. It’s the same as you extending your right hand and your friend extending his right hand at the same time. You can’t shake hands because your hands are in the wrong position.
Similarly, water and oil can’t “shake hands" or have a chemical reaction because their molecules are in the wrong position to mix, even though the molecules are still moving around (even if you and your friend started moving your hands, shaking hands wouldn’t be possible without twisting around completely).
So What Causes a Violent Chemical Reaction?
Here’s the final piece of the puzzle of what causes chemical reactions. Our volcano erupts not only because the vinegar and baking soda were able to “shake hands" with each other but also because they released energy at the same time. Certain molecules, when they chemically react, don’t do so easily. They combine, but because one may be moving at different speeds than the other, they combine violently.
To understand what causes a chemical reaction like this, imagine (but don’t try it – you could hurt yourself) that you and your friend were bumping fists. You thrust your fist out like a lightning bolt while he was extending his fist slowly. If your fists collided with each other in mid air, the two of you would hurt each other because of the difference in energy levels. Similarly, the different energy levels of the chemicals can cause violent chemical reactions to occur when they combine.