Balance equations and classify reaction types — synthesis, decomposition, single and double replacement. Balancing is really just conservation of mass, written out atom by atom. Every question comes with a written walkthrough of the balancing or classification.
Changing a subscript, like H₂O to H₂O₂, creates a different substance entirely — only the coefficient in front of a formula can be adjusted when balancing.
In 2H₂O, the coefficient 2 multiplies everything in the formula — that's 4 hydrogen atoms and 2 oxygen atoms total, not 2 hydrogen and 2 oxygen.
Single and double replacement reactions can both start with two reactants and end with two products — the real distinguishing factor is whether one element swaps into a compound or two compounds trade ions, not the count alone.
Straight from the bank — one per difficulty tier. Reveal the answer to see the explanation you'd get in a real session.
Balance the following equation: __H₂ + __O₂ → __H₂O
A — 2H₂ + O₂ → 2H₂O. Balancing requires equal atoms of each element on both sides: 2H₂ gives 4 hydrogen atoms, O₂ gives 2 oxygen atoms, and 2H₂O gives 4 hydrogen and 2 oxygen atoms — both sides match. H₂ + O₂ → H₂O (B) leaves oxygen unbalanced: 2 oxygen atoms on the left but only 1 on the right.
Identify the type of reaction: 2Mg + O₂ → 2MgO
A — Synthesis. Two simpler substances, magnesium and oxygen, combine to form a single more complex product, magnesium oxide — that's a synthesis reaction. Decomposition (B) would go the other direction, starting with a single compound and breaking it apart into simpler substances.
Identify the type of reaction: AgNO₃ + NaCl → AgCl + NaNO₃
A — Double replacement. Two compounds, AgNO₃ and NaCl, exchange ions — silver pairs with chloride and sodium pairs with nitrate — producing two new compounds. That exchange between two compounds is a double replacement reaction. Single replacement (B) would involve one free element displacing another element within a single compound, not two compounds trading ions with each other.
Balancing and classifying reactions are two separate skills tested together — balancing is arithmetic, classification is pattern recognition, and mixing up the approach for each slows both down.
Write out the atom count for each element on both sides of the equation before changing anything. Untimed practice is where that counting habit becomes reliable.
Move to timed sessions once balancing and classification are both fast.
Pair chemical reactions with gas laws in a mock — many gas-law problems start from a balanced chemical equation.
Already have an account? Sign in →