Synthesis and decomposition — decomposition deep-dive and balancing

Last lesson you met three kinds of decomposition — thermal, electrical, photochemical. Today we go deeper using the textbook's lead example: the decomposition of hydrogen peroxide (H₂O₂).

Once you've written your three predictions, watch the real-world demo — fresh liver dropped straight into hydrogen peroxide. Compare what you see against what you predicted before going into Explicit.

And here's the same reaction with potato instead of liver. Both work because both tissues contain the enzyme catalase — a protein found in nearly all living cells (animal and plant), where its everyday job is to break down stray H₂O₂ that builds up during cellular respiration. The chemistry is identical: catalase from any source speeds the same H₂O₂ → H₂O + O₂ reaction.

Quick guess — if the potato had been cooked first (boiled, baked, or fried), would you still see vigorous foaming when it hits the H₂O₂? Decide yes / no / less in your head before revealing.

Now to a very different kind of decomposition — driven not by an enzyme, but by electricity. Open the OLabs simulation below on your laptop, choose Electrolytic Decomposition from the dropdown and Water as the sample, then switch on the 12 V battery. Watch the two test tubes for about thirty seconds.

If your laptop won't run the sim, use the snapshot above as your observation. Otherwise compare what you actually saw to the options below:

Now look at the balanced equation for the electrolysis of water:

The equation says: every 2 molecules of H₂ produced come paired with only 1 molecule of O₂ — a 2 : 1 ratio of molecules. You just observed a 2 : 1 ratio of gas volumes. Use the equation to figure out which gas is in which tube:

You just used the coefficients in a balanced equation to predict an experimental observation — that's the power of balancing equations. In Explicit, you'll learn how to write a balanced equation yourself when the coefficients aren't already given to you.

What actually happens in the beaker

When liver (which contains the enzyme catalase) drops into a hydrogen peroxide solution, three pieces of evidence tell us a chemical reaction occurred:

1. Gas released — vigorous bubbling at the surface of the liver; the bubbles are pure oxygen (you can test by relighting a glowing splint).
2. Heat released — the beaker warms up; the reaction is exothermic.
3. Liquid left behind — as the H₂O₂ is used up, the solution becomes plain water.

Word form: hydrogen peroxide → water + oxygen.

One reactant → two products. That's the signature of decomposition from last lesson. Catalase sits above the arrow as a catalyst — it speeds up the reaction without being consumed. (Without a catalyst, H₂O₂ still decomposes on its own, just much more slowly — which is why old bottles of peroxide on a shelf lose strength over time.)

Why the atoms balance — conservation of mass

Here is the single most important idea in the whole topic:

In a chemical reaction, atoms are neither created nor destroyed — they are rearranged. So the number of each type of atom on the LEFT of the arrow must equal the number on the RIGHT.

This is the Law of Conservation of Mass. It's why a balanced equation is so useful: it's a true statement about atom bookkeeping.

Let's check 2H₂O₂ → 2H₂O + O₂:

The tools you use to balance — coefficients (not subscripts)

A coefficient is the BIG number in front of a formula. A subscript is the small number inside a formula. You balance by changing coefficients, NEVER subscripts — changing a subscript would change what substance you have.

Worked example — the electrolysis of water from last lesson:

• H: left = 2 × 2 = 4; right = 2 × 2 = 4. Match.
• O: left = 2 × 1 = 2; right = 1 × 2 = 2. Match.

The coefficient "2" in front of H₂O means two molecules of water. The subscript "2" inside H₂O means each water molecule has two hydrogen atoms. Different jobs, different numbers.

State symbols — the tiny letters in brackets

A proper chemical equation also tells you what physical state each substance is in. This matters because the same compound can do different chemistry depending on its state.

Three decomposition examples — every one balanced, every one with states

Notice the general pattern for a metal-carbonate decomposition (rows 2 and 3): metal carbonate → metal oxide + carbon dioxide. Once you spot the pattern you can predict the products without memorising each one.

Key terms

Work through all four questions. Each one is a different format so you have to think, not just click.

One short application before you go — closes the loop on today's catalyst story.

Learning Ladder — where are you right now?

Next class (Wed 29 Apr, P5): we start on the other big reaction type — displacement. You'll meet the activity series of metals and learn to predict which metal will "kick out" another from a solution.