RxnWorks

Chemical change, balancing, reaction types, solubility, net ionic equations and acid-base reactions.
Read the idea in Learn, then practise. Each tab has Hint, Reset and Skip, 3 levels and a crown to win.

Track 0 · Introductory Chemistry · part of ChemTracks

Start here: what happens in a reaction?

In a chemical reaction, atoms do not appear or disappear: they rearrange into new substances. An equation is the recipe, and each tab here teaches one skill of reading or writing it. Work from the top: the steps build on each other.

Build it, test it, own it

Read the idea, then practise it. Every tab has Hint, Reset and Skip, 3 levels, and a crown to win. You never need to memorize: you will meet each idea again and again until it is yours.

Sense check

Before you press Check, ask: does this make chemical sense? Is the atom count the same on both sides? Is the product one that could really form?

Step 1. Chemical change and reading an equation

A chemical change makes a new substance. Clues: gas bubbles, a solid forming from two solutions, a color change, heat or light given off. In an equation, reactants (left) turn into products (right). The coefficient is the big number in front; the subscript is the small number inside the formula. State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.

Worked example. In 3 Ca(NO₃)₂, each unit has 1 Ca, 2 N and 6 O (the 2 outside the parentheses multiplies everything inside). With the coefficient 3: 3 Ca, 6 N and 18 O.

Step 2. Balancing equations

Matter is conserved, so every element must have the same number of atoms on both sides. Change only the coefficients, never the subscripts. Balance the elements that appear in one substance on each side first; leave H and O (and any lone element) for last. If you reach a fraction, double everything. Finish in lowest terms.

Worked example. H₂ + O₂ → H₂O has 2 O on the left and 1 on the right. Put a 2 in front of H₂O: now 4 H on the right, so put a 2 in front of H₂. 2 H₂ + O₂ → 2 H₂O: 4 H and 2 O on each side.

The two nevers of chemistry

NEVER work with mass ratios, only with mole ratios.

NEVER work with unbalanced reactions, always balance first.

Step 3. Five types of reactions

Type	Pattern
Synthesis	A + B → AB
Decomposition	AB → A + B
Single replacement	A + BC → AC + B
Double replacement	AB + CD → AD + CB
Combustion	fuel + O₂ → CO₂ + H₂O

Worked example. Zn + CuSO₄ → ZnSO₄ + Cu. An element replaces another in a compound, so this is single replacement. It happens because Zn sits above Cu in the activity series (Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag): a more reactive metal replaces a less reactive one.

Step 4. Solubility and precipitates

Some ionic compounds dissolve in water; others stay together as a solid. When two solutions mix, the ions can swap partners. If a new insoluble compound forms, it appears as a solid called a precipitate.

Ion	Rule
Group 1 and NH₄⁺	always soluble
NO₃⁻	always soluble
Cl⁻, Br⁻, I⁻	soluble, except Ag⁺ and Pb²⁺
SO₄²⁻	soluble, except Ba²⁺, Pb²⁺, Ca²⁺
OH⁻	insoluble, except Group 1 and Ba²⁺
CO₃²⁻, PO₄³⁻, S²⁻	insoluble, except Group 1 and NH₄⁺

Worked example. AgNO₃(aq) + NaCl(aq): swap partners to get AgCl and NaNO₃. Nitrates are soluble, but chlorides of Ag⁺ are not, so AgCl is the precipitate.

Step 5. Ionic and net ionic equations

Soluble ionic compounds exist as separate ions in water, so split every (aq) ionic compound into its ions and keep solids, liquids and gases whole. Ions that appear unchanged on both sides are spectator ions: cancel them. What is left is the net ionic equation, the real chemistry.

Worked example. Pb(NO₃)₂(aq) + 2 KI(aq) → PbI₂(s) + 2 KNO₃(aq).
Spectators: K⁺ and NO₃⁻.
Net ionic: Pb²⁺(aq) + 2 I⁻(aq) → PbI₂(s). Atoms and charges balance (2+ plus 2×1− = 0).

Step 6. Acids, bases and neutralization

An acid donates H⁺ (a proton); a base accepts it. An acid that loses H⁺ becomes its conjugate base; a base that gains H⁺ becomes its conjugate acid. Strong acids (HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄) ionize completely. Neutralization: acid + base → salt + water.

Worked example. H₂SO₄ + 2 NaOH → Na₂SO₄ + 2 H₂O. The salt is the metal from the base (Na⁺) with the anion from the acid (SO₄²⁻). Conjugate base of HSO₄⁻: remove H⁺ to get SO₄²⁻.

Practice chemical reactions

RxnWorks is Track 0 of ChemTracks. Each topic has a short lesson, a practice tab with three levels of randomly generated problems, hints and worked solutions, and a crown for mastering it. Earn all seven crowns for a certificate.

Topics covered

Physical and chemical change, reading an equation, counting atoms
Balancing chemical equations
Synthesis, decomposition, single and double replacement, combustion, and the activity series
Solubility rules and predicting precipitates
Complete and net ionic equations and spectator ions
Acids, bases, neutralization and conjugate pairs
The Reaction Lab challenge

Frequently asked questions

What does RxnWorks cover?

Chemical change and reading equations, balancing equations, the five reaction types, solubility rules and precipitates, ionic and net ionic equations, and acids, bases and neutralization.

How do I balance a chemical equation?

Change only the coefficients, never the subscripts. Count each element on both sides, balance elements that appear in one substance first, leave H and O for last, and finish in lowest terms.

Which track is RxnWorks in?

Track 0 (Introductory Chemistry) of ChemTracks. Oxidation numbers, redox and predicting products come later in Track 1.

Do I get a certificate?

Yes. Earn every crown in RxnWorks and you can claim a free certificate of completion as a PDF. Finish all six Track 0 Works to also earn the Track 0 badge and certificate, which anyone can verify.

Do I need an account to use RxnWorks?

No. It runs in your browser and saves your progress on your own device.