A chemical change forms a new substance — but how can you tell?
Chemical Change Clues
A chemical change is any change in which a new substance is formed. You can't always see the molecules,
but five kinds of evidence tip you off. Hover a clue to highlight it.
Chemical Change Clues
Spot the evidence
Watch each scenario, then click which type of evidence you observe.
Chemical Equations & Conservation of Mass
Reactants are used up · products are created · matter is never lost
What a chemical equation shows
A chemical equation represents a reaction using: chemical formulas,
the relative number of entities (coefficients), and the states of matter.
A reactant is a chemical used up in the reaction.
A product is a chemical created during the reaction.
2H2O → 2H2 + O2
↑ reactantproducts ↑
Law of Conservation of Mass
The total mass before a reaction equals the total mass after.
Matter cannot be created nor destroyed — so every atom on the left must reappear on the right.
That's why we balance equations.
Cu + AgNO3 → Ag + Cu(NO3)2
Balancing Chemical Reactions
Add coefficients until every element's atom count matches on both sides
Interactive balancer
Presets from the slides
Strategy tips for balancing
Balance compounds first, elements last.
Only add coefficients — NEVER change subscripts.
Balance metals first, then nonmetals, then H & O.
If H and O appear in more than one place, balance them last.
Multiply coefficients × subscripts to count atoms.
Polyatomic ions (SO42−, OH−, NO3−) can be balanced as a whole group.
If even numbers don't work as coefficients, try odd numbers.
Use diagrams to help if you're stuck.
Balanced Equations from Word Equations
Word equation → skeleton (correct formulas) → balanced
Three-step converter
Word equation
Type -> or → for the arrow. Subscripts and coefficients are just normal numbers (H2O, 2H2O).
Elements combine (synthesis) · a compound splits into elements (decomposition)
Simple synthesis
Two elements combine to form one compound: A + B → AB.
If one reactant is O2, it's also combustion (e.g. 2Mg + O2 → 2MgO).
Simple decomposition
A compound breaks down into its elements: AB → A + B.
Predict the product
Synthesis
Balanced products:
Predicting Products — Single Displacement
A + BC → AC + B · only if A is more reactive than B
Activity series (reference)
most reactive → least reactive
Metal activity series
Halogen activity series
A reaction only occurs if the free element is higher (more reactive) than the one it's trying to replace. The zones show what each metal reacts with.
"Will it react?" simulator
Metals reacting with acids
Many metals react with acids to give a salt + hydrogen gas:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g).
Metals below hydrogen (Cu, Hg, Ag, Au) do not react — the answer is NR.
Predicting Products — Double Displacement
AD + BC → AC + BD · the positive ions swap partners
Acid–base (neutralization)
acid + base → salt + water, always.
NaOH + HCl → NaCl + H2O
Precipitation
two aqueous solutions → a low-solubility precipitate (s).
2NaOH + CuCl2 → 2NaCl + Cu(OH)2(s)
Ion-swap predictor
Hydrocarbon Combustion
Fuel + O2 → complete: CO2 + H2O · incomplete: + CO + C
Combustion simulator
Combustion type
Products
Balanced (complete)
Complex Synthesis & Decomposition
Four matching pairs — each synthesis pattern reverses to a decomposition
Pattern flashcards (click to flip)
Test yourself
Predict the product for each pattern.
Double Displacement Reactions Forming a Gas
Some double-displacement products are unstable and break down into a gas + water
The three unstable products
If a DD reaction forms…
It breaks down into…
H2CO3 (carbonic acid)
CO2(g) + H2O(l)
H2SO3 (sulfurous acid)
SO2(g) + H2O(l)
NH4OH (ammonium hydroxide)
NH3(g) + H2O(l)
Two-step walkthrough
State Symbols
(s) solid · (l) liquid · (g) gas · (aq) aqueous
General rules
Metals are solid (s) — except Hg is liquid (l).
Water is liquid (l) — except in combustion it's usually gas (g).
Common gases: O2, CO2, CO, NH3.
HOFBrINCl diatomics are all gases — except Br2 is liquid, I2 is solid.
Ionic compounds tend to be solid; covalent can be s, l, or g.
Acids are always aqueous (aq); ionic compounds dissolved in water are (aq).
State symbol drill
Chemical Reactions Kahoot
28 questions · 20 seconds each · Speed bonus scoring
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Ready to test your knowledge?
Covers evidence, balancing, the five reaction types, predicting products, activity series & state symbols