In any chemical reaction, atoms aren't created or destroyed; they just get rearranged. This is the law of conservation of mass. To respect this law, we must balance chemical equations. A balanced equation has the same number of atoms of each element on both the reactant (starting materials) and product (what's formed) sides.

Think of it like a recipe. If you need two cups of flour and one cup of sugar to make a cake, you can't just throw in one cup of each and expect the same result. The proportions matter. In chemistry, these proportions are given by stoichiometric coefficients, the numbers we place in front of chemical formulas.

Let’s take the formation of water from hydrogen and oxygen. The initial, or skeleton, equation looks like this:

Notice the problem? We have two oxygen atoms on the left but only one on the right. To fix this, we adjust the coefficients. We can't change the subscripts (the little numbers like the 2 in $H_2O$), because that would change the substance itself. Changing $H_2O$ to $H_2O_2$ would mean we're making hydrogen peroxide, not water.

The correct, balanced equation is:

Chemical reactions come in many forms, but most can be sorted into a few main categories. Understanding these types helps us predict what will happen when substances are mixed.

Combination Reactions: Two or more simple substances combine to form a more complex product. Think of it as a partnership forming. A + B → AB.

Decomposition Reactions: A single compound breaks down into two or more simpler substances. This is like a partnership dissolving. AB → A + B. This often requires energy in the form of heat, light, or electricity.

Displacement Reactions: One element takes the place of another element in a compound. A more reactive element kicks out a less reactive one. A + BC → AC + B.

Double Displacement Reactions: The positive and negative ions of two ionic compounds switch places to form two new compounds. It's like two dancing couples swapping partners. AB + CD → AD + CB. These often result in the formation of a precipitate, an insoluble solid.

A great example of a double displacement reaction is when you mix solutions of lead(II) nitrate and potassium iodide. A vibrant yellow solid, lead(II) iodide, instantly forms and settles at the bottom. This is a precipitate reaction, a hallmark of this reaction type.

Many chemical reactions, including some we've already discussed, are driven by the transfer of electrons from one atom to another. These are called oxidation-reduction reactions, or redox reactions for short.

Here's the key: Oxidation Is Loss of electrons, and Reduction Is Gain of electrons. A handy mnemonic is "OIL RIG".

• When an atom loses electrons, its oxidation state (a sort of charge counter) increases. It has been oxidised.
• When an atom gains electrons, its oxidation state decreases. It has been reduced.

Crucially, oxidation and reduction always happen together. If one substance loses electrons, another must gain them. The substance that gets oxidised is called the reducing agent, because it causes the reduction of the other substance. Similarly, the substance that gets reduced is the oxidising agent.

Consider the simple reaction between sodium (Na) and chlorine (Cl) to form table salt (NaCl):

In this reaction, each sodium atom loses one electron to become a positive ion ($Na^+$), so sodium is oxidised. Each chlorine atom in the $Cl_2$ molecule gains one electron to become a negative ion ($Cl^−$), so chlorine is reduced. Sodium is the reducing agent, and chlorine is the oxidising agent.

You don't need a lab to see redox reactions. Two common examples are the rusting of iron and the spoilage of food.

Corrosion: When iron is exposed to oxygen and water, it rusts. This is the slow oxidation of iron metal. The iron atoms lose electrons to oxygen, forming iron oxides like $Fe_2O_3$. This process weakens the metal, causing billions of dollars in damage to infrastructure every year.

Rancidity: This is what happens when fats and oils in food are exposed to air. The fatty acids are oxidised, which changes their chemical structure and leads to the unpleasant smells and flavours of spoiled food. To prevent this, manufacturers often add antioxidants to food products. These are substances that are more easily oxidised than the fats, so they 'sacrifice' themselves to protect the food.

Understanding these fundamental reaction types and the electron transfers that drive them is the key to predicting and controlling chemical changes all around us.

Time to check your understanding of these core chemical concepts.