An acid is a substance that ionises in water to produce hydrogen ions, H⁺, so its acidic properties are shown only in the presence of water.
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An acid is a substance that ionises in water to produce hydrogen ions, H⁺. The whole definition hangs on those last words: an acid shows its acidic properties only in the presence of water, because it is the free hydrogen ions released into the solution that make a substance behave as an acid. Write the phrase “in water” every time, since the marking scheme rewards it and quietly withholds the mark when it is missing.
A worked example makes this concrete. Hydrogen chloride gas, HCl, does not turn dry blue litmus red and does not react with a metal while it stays dry. Dissolve it in water and it ionises, HCl → H⁺ + Cl⁻, and the solution is now hydrochloric acid: it turns blue litmus red, reacts with magnesium to give hydrogen gas, and neutralises alkalis. The same molecule is inert dry and strongly acidic once water sets its hydrogen ions free. Ethanoic acid in vinegar and sulfuric acid in a car battery behave the same way for the same reason.
The confusion to avoid is treating “acid” and “hydrogen” as loose synonyms. Many compounds contain hydrogen, methane, CH₄, and ammonia, NH₃, yet they are not acids, because their hydrogen is not released as H⁺ ions in water. Only the hydrogen that ionises off counts. A second slip is to leave out “in water” and define an acid merely as “a substance that produces hydrogen ions”, which loses the point about the role of water.
Within this chapter the acid is the counterpart of the base and alkali: acids give H⁺, alkalis give OH⁻, and their reaction is neutralisation. Because SPM Chemistry (4541) tests this definition directly, our teachers drill the exact wording until you can write it without hesitation.
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