ACARA v9 CONTENT DESCRIPTION “model the rearrangement of atoms in chemical reactions using a range of representations, including word and simple balanced chemical equations, and use these to demonstrate the law of conservation of mass”
Builds on the idea that a chemical change makes a new substance. Here we look inside that change: the atoms in the reactants are only rearranged into the products, never made or lost. That single fact lets us write balanced equations and explains why mass is conserved.
Reactions rearrange atoms
In a chemical reaction the starting substances, the reactants, turn into new substances, the products. Zoom in and you see why: bonds between atoms break and the same atoms re-join in a new pattern. Take two hydrogen molecules and one oxygen molecule, written 2H2 + O2. The reaction regroups those four hydrogen atoms and two oxygen atoms into two water molecules, 2H2O. The atoms are the same; only their partners have changed.
Atoms rearrange, they are never lost
Step the reaction forward. The old bonds break and the very same atoms re-join in a new pattern.
We start with the reactants: two hydrogen molecules and one oxygen molecule. Count the atoms now: four H and two O.
Word equations
Before we reach for symbols we can describe a reaction in plain words. A word equation names the reactants on the left, an arrow that reads reacts to make in the middle, and the products on the right. Methane + oxygen reacts to make carbon dioxide + water. Word equations tell us what reacts and what forms, which is the first representation of a reaction.
Build a word equation
Pick a reaction. Reactants go on the left, the arrow means reacts to make, and the products go on the right.
The fuel methane reacts with oxygen from the air. The arrow means "reacts to make": the products are carbon dioxide and water.
Balancing a symbol equation
A symbol equation uses formulas instead of names, so it can be checked atom by atom. Writing H2 + O2 -> H2O looks neat but is not balanced: the left has two oxygen atoms and the right only one. We fix this with coefficients, the big numbers placed in front of each formula, never the small subscripts inside. The balanced version is 2H2 + O2 -> 2H2O, which has four hydrogen and two oxygen atoms on each side.
Balance the equation
Raise and lower the coefficients until the atom count on the left matches the right for every element.
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Not balanced. We may only change the big numbers in front (the coefficients), never the small ones inside a formula, because changing those would make a different substance.
Counting the atoms
A balanced equation is a promise that every element is accounted for. To check it, tally each element separately and compare the two sides. In 2H2 + O2 -> 2H2O there are four hydrogen atoms on the left and four on the right, and two oxygen atoms on the left and two on the right. When every element matches, the equation is balanced.
Tally the atoms on each side
Choose an element and count its atoms. In a balanced equation the left stack always equals the right stack.
Hydrogen: the two H2 molecules on the left give 4 H atoms, and the two H2O molecules on the right also hold 4 H atoms. They match.
The law of conservation of mass
Because no atoms are created or destroyed, the total mass of the products equals the total mass of the reactants. This is the law of conservation of mass. It only appears to fail when a gas escapes or enters an open container; carry out the same reaction in a sealed flask and the balance stays perfectly level. Every atom that goes in comes out, so the mass is conserved.
Conservation of mass
Run the reaction on a beam balance. In a sealed flask no atoms leave, so the total mass never changes.
Before the reaction the reactants sit on the left pan with a total mass of 36 units. This is our starting point.
Why this matters
Balanced equations and conservation of mass are the bookkeeping of all chemistry. They let us predict how much of each substance is needed and how much will form, from the fuel in an engine to the medicines in a lab. Once you trust that atoms are only rearranged, every reaction becomes a careful regrouping you can count.
Quick self-check
1. During a chemical reaction, what happens to the atoms?
2. In the word equation iron + oxygen reacts to make iron oxide, which are the products?
3. When balancing H2 + O2 -> H2O, which numbers are you allowed to change?
4. A balanced equation has 4 hydrogen atoms on the left. How many hydrogen atoms are on the right?
5. You react substances in a sealed flask. How does the total mass after compare with before?