At its heart, chemistry is the study of change. A chemical reaction is a process where one or more substances, called reactants, are converted into one or more different substances, known as products. It's not about substances disappearing, but about their atoms rearranging to form new things.

Think about baking a cake. You start with flour, eggs, sugar, and butter. These are your reactants. After mixing and baking, you have a cake—the product. The cake has completely different properties from the raw ingredients, but it's made from the same atoms, just rearranged into new molecules.

A classic chemical example is the rusting of an iron nail. The iron in the nail reacts with oxygen from the air. The result is a flaky, reddish-brown substance called rust, or iron oxide. The nail you started with and the rust you end up with are fundamentally different materials.

The original substances are the reactants, and the new substances formed are the products.

Chemists need a clear, concise way to describe reactions. Instead of writing long sentences, they use chemical equations. A chemical equation is like a recipe that lists the reactants and the products.

The arrow indicates the direction of the reaction, showing that the reactants are transformed into the products. We can start by writing this out in words. For the formation of water, the word equation is:

Hydrogen + Oxygen → Water

To be more specific, chemists use chemical formulas for each substance. Hydrogen gas is $H_2$, oxygen gas is $O_2$, and water is $H_2O$. This gives us a skeleton equation:

This equation tells us what’s involved, but it doesn't tell the whole story. It's missing a key piece of information about the amounts of each substance, which relates to a fundamental law of nature.

The most important rule in chemical reactions is the Law of Conservation of Mass. It states that matter is not created or destroyed in any chemical reaction. The atoms you start with are the same atoms you end with; they’re just rearranged.

Let’s look at our skeleton equation for water again: $H_2 + O_2 \rightarrow H_2O$. On the reactant side (left of the arrow), we have two hydrogen atoms and two oxygen atoms. On the product side (right of the arrow), we have two hydrogen atoms but only one oxygen atom.

An oxygen atom seems to have vanished, which violates the Law of Conservation of Mass. This tells us the skeleton equation is incomplete. We need to account for all the atoms, ensuring the same number of each type of atom appears on both sides of the arrow. This process is called balancing the equation, and it ensures the law is obeyed.

The image above shows the balanced equation. Notice how there are four hydrogen atoms and two oxygen atoms on both sides. They've just been rearranged to form two water molecules. Understanding this principle is the first step toward predicting the outcomes of chemical reactions.

Time to check your understanding.

Now you know the basics of what happens during a chemical reaction and how chemists represent these changes. The Law of Conservation of Mass is the guiding principle for all of chemistry.