Describe acids and bases using the Arrhenius and Bronsted-Lowry definitions, identify their characteristic properties, and write simple neutralization reactions.
The Arrhenius definition, the simplest starting point, defines an acid as a substance that produces H+ ions (more precisely, H3O+, hydronium ions) when dissolved in water, and a base as a substance that produces OH- (hydroxide) ions when dissolved in water. The broader Bronsted-Lowry definition describes acids as proton (H+) donors and bases as proton acceptors, which usefully extends the idea to reactions that do not even involve water.
Acids share a recognizable set of properties: a sour taste (never taste unknown lab chemicals -- this is known from safely edible acids like citric acid in citrus fruit and acetic acid in vinegar), the ability to turn blue litmus paper red, reaction with many metals to release hydrogen gas, reaction with carbonates to release carbon dioxide gas, and a pH below 7. Bases share their own set: a bitter taste and slippery feel (again, known from safe household examples like soap), the ability to turn red litmus paper blue, and a pH above 7. Common acids include hydrochloric acid (found naturally in stomach acid) and sulfuric acid (used in car batteries); common bases include sodium hydroxide (drain cleaner) and ammonia (many household cleaners).
When an acid and a base react, they neutralize each other, producing a salt (an ionic compound) and water: acid + base -> salt + water, as in HCl + NaOH -> NaCl + H2O. This reaction is the basis of titration, a lab technique where a solution of known concentration is carefully added to a solution of unknown concentration, drop by drop, until an indicator shows the reaction is exactly complete -- allowing chemists to calculate the unknown concentration from the amount of known solution it took to neutralize it.
Mastery looks like: They correctly classify a substance as an acid or base using its formula or properties, and correctly write the products of a simple neutralization reaction.
Common stumbles: Confusing which litmus color change signals acid versus base; forgetting that a neutralization reaction always produces water in addition to a salt.