Mistake Master
Introduction to reactions
Ice melting and iron rusting both change what you see, but only one makes a new substance. A chemical reaction rearranges atoms into something new, and because atoms are never lost, its equation has to balance.
§1
What a reaction is.
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A chemical reaction rearranges atoms into new substances with different properties. A physical change — melting, boiling, dissolving — changes form or state but leaves the substance's identity the same.
Atoms are conserved: none are created or destroyed in a reaction, only rearranged. That is why a reaction is written as a balanced equation, with the same number of each kind of atom on both sides.
Signs like bubbling, heat, or color can hint at a reaction, but none of them proves one by itself — dissolving fizzes, boiling bubbles. The real test is whether the composition changed into new substances.
§2
Recognizing and balancing a reaction.
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Judge by composition, then balance by conserving atoms.
- Ask whether new substances formed. If the composition changed into different substances, it is a chemical reaction; if not, it is physical.
- Do not rely on surface signs alone. Bubbles, heat, or color changes accompany some reactions but also some physical changes; they are hints, not proof.
- Balance by conserving atoms. Adjust coefficients so each element has equal atoms on both sides. Never change subscripts.
- Check every element. Count each element on the left and right; they must match for the equation to be valid.
§3
The pieces you'll meet.
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A few ideas anchor the whole topic.
§4
Worked example: balance the burning of methane.
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Question. Balance CH₄ + O₂ → CO₂ + H₂O.
Carbon. One C on each side already balances.
Hydrogen. CH₄ has 4 H; put a 2 in front of H₂O to give 4 H on the right.
Oxygen. The right now has 2 (from CO₂) + 2 (from 2 H₂O) = 4 O; put a 2 in front of O₂. Final: CH₄ + 2O₂ → CO₂ + 2H₂O, with every element balanced and no subscripts changed.
§5
Mistakes that cost real points.
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"Bubbles prove a chemical change happened."
Bubbles can come from a physical change too — boiling water bubbles, and a dissolving gas fizzes — without any new substance forming. Bubbling is a hint, not proof. The real test is whether the composition changed.
Fix. Judge by composition: did new substances form? Use bubbles only as a clue to investigate, not as a verdict.
"An equation does not really need to be balanced to be correct."
A chemical equation must be balanced because atoms are conserved. An unbalanced equation claims atoms appeared or vanished, which cannot happen. The coefficients exist precisely to make the atom counts match.
Fix. Always balance so each element has equal atoms on both sides. Adjust coefficients, never subscripts.
"Dissolving a substance destroys its molecules."
Dissolving is a physical change: the substance's particles are dispersed among the solvent, not destroyed. Dissolved sugar is still sugar molecules; dissolved salt is still Na⁺ and Cl⁻ ions. Evaporate the water and it comes back.
Fix. Treat dissolving as spreading particles apart, not destroying them. The substance can be recovered unchanged.
§6
Skill Check.
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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.