Chemical reactions: Introduction

If you're performing a magic trick, you can make objects disappear... ... or appear. Of course, that's just a trick, we can't really make things disappear or appear. Or can we? Let's do a similar magic trick, but this time using two gases: hydrogen and oxygen.

Instead of waving a magic wand, you use a burning match. Abraca... dabra! The substances we started out with have disappeared, and we've conjured - water! Unlike the previous magic trick, the gases really did disappear.

And the water wasn't already there from the start. Chemistry can seem like magic sometimes: You make the starting substances disappear, and make other substances appear. How is that possible? How can substances cease to exist, and new substances turn up? To answer that question, we have to get closer - much closer - and look at what we mean by a "substance".

All substances consist of atoms. Hydrogen gas is made of hydrogen atoms, and oxygen gas is made of oxygen atoms. It's the atoms involved, and how they're attached, that determines the substance. In these substances, the atoms are attached two by two. If we mix the gases, and set fire to them, the atoms will let go of each other, and be re-arranged.

Now, each oxygen atom is attached to two hydrogen atoms instead. The atoms are the same, but when they are attached like this, they form a different substance than those we started with. A chemical reaction has taken place. The atoms are still there, but it isn't hydrogen gas anymore, nor oxygen gas. When the atoms are attached to each other in a different way than before, they form a new substance - in this case, water.

In a chemical reaction, atoms will detach from each other, attach to each other, or both. Does that mean that every time a bond between atoms is broken or formed, a chemical reaction has taken place? Let's look at the water again. When hydrogen atoms and oxygen atoms are arranged this way, they have formed water molecules. If we cool the water, so it freezes, the molecules will attach to each other.

New bonds have formed between the atoms of different molecules. Does that mean water freezing to ice is a chemical reaction? Actually, it isn't. Why not? Because each molecule still looks the same, with one oxygen atom and two hydrogen atoms.

That means we still have the same substance - water. Only now it's in solid form. In order to change it into something other than water, we have to change what the molecules look like. For instance, we can add a piece of this metal: sodium. The sodium reacts with the water, splitting the water molecules.

One hydrogen atom is disconnected from the rest of the molecule. The freed hydrogen atoms attach two by two, forming a new substance: hydrogen gas. The part of the water molecule that remains, is one oxygen atom and one hydrogen atom. Together with the sodium, they form a new substance - sodium hydroxide. This is an example of a chemical reaction with water.

The water molecules have been divided, and two new substances have been formed: hydrogen gas and sodium hydroxide. To summarize: In a chemical reaction, substances can cease to exist, and new substances can form. The atoms that compose those substances are still the same. They have just moved around and attached in different ways. It's not magic.

It's chemistry.