By this point you've learned five reaction type families: synthesis, decomposition, single displacement, double displacement, and combustion. The real challenge on a test is being handed a reaction you've never seen and figuring out, from the reactants alone, which pattern applies and what the products will be. Chemists do not memorize thousands of separate reactions; they recognize a handful of patterns and reason from them. A few very common, very predictable patterns involving acids are worth knowing by heart, because they show up everywhere from your stomach to the kitchen to the weathering of buildings.
Acid–base neutralization
An acid and a base react to form a salt (an ionic compound) and water: acid + base → salt + water. This is technically a double displacement reaction: the H⁺ from the acid pairs with the OH⁻ from the base to form H₂O, while the remaining ions combine to form the salt. Forming water is what drives the reaction, since it pulls and out of solution as neutral molecules. Antacid tablets containing magnesium hydroxide work exactly this way on excess stomach acid (HCl): To write the salt, take the metal cation from the base and the anion from the acid, then balance charges.
Acid + carbonate
An acid reacting with a metal carbonate produces a salt, water, and carbon dioxide gas: acid + metal carbonate → salt + water + CO₂(g), recognizable by the fizzing/bubbling as CO₂ escapes. Why three products? It is really two reactions in a row. First a double displacement swaps partners and forms carbonic acid, . Carbonic acid is unstable and immediately decomposes into water and carbon dioxide. Baking soda and vinegar foaming, limestone and marble statues slowly dissolving in acid rain, and the geology test of dripping acid on a rock to see if it fizzes are all this pattern. Hydrogen carbonates such as behave the same way.
Acid + metal
This is simply a single displacement reaction: a reactive metal (above hydrogen on the activity series) displaces hydrogen from an acid, forming a salt and hydrogen gas. The metal gives electrons to the ions, which pair up as molecules. A burning splint held at the mouth of the test tube gives a squeaky pop, the classic test for hydrogen. Copper, silver and gold sit below hydrogen, so they give no bubbles. Contrast this with the carbonate reaction: both fizz, but hydrogen pops a burning splint while carbon dioxide puts the flame out.
Combustion patterns, revisited
Whenever you see a hydrocarbon reacting with O₂, recognize combustion immediately: complete combustion (excess O₂) gives CO₂ + H₂O; incomplete combustion (limited O₂) gives CO + H₂O, or soot in severe cases. A metal plus O₂, on the other hand, is synthesis of a metal oxide, not combustion in the hydrocarbon sense, even though magnesium burns with a dazzling flame.
| Reactant clue | Reaction type | Product pattern |
|---|---|---|
| Metal + nonmetal (elements) | Synthesis | Ionic compound |
| Metal oxide or nonmetal oxide + water | Synthesis | Base (metal hydroxide) or acid |
| Very active metal (Li to Na) + water | Single displacement | Metal hydroxide + H₂(g) |
| One compound, heated/electrolyzed | Decomposition | Simpler substances (e.g. oxide + CO₂, or elements) |
| Element + compound | Single displacement | Check activity series — new compound + displaced element, or NR |
| Two ionic (aq) compounds | Double displacement | Check solubility rules — precipitate, or NR |
| Acid + base | Neutralization (double displacement) | Salt + water |
| Acid + carbonate | Double displacement | Salt + water + CO₂(g) |
| Acid + metal | Single displacement | Salt + H₂(g) (only if metal is above H) |
| Hydrocarbon + O₂ | Combustion | CO₂ + H₂O (excess O₂) or CO/soot + H₂O (limited O₂) |