To safely dilute a strong concentrated acid, the acid must always be added slowly to water with constant stirring, and never the other way round.
Reason: Dissolving an acid in water is a highly exothermic process. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating.
Source: Chapter 2, Section 2.2.1
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Examiners expect two things: (1) the correct procedure — acid added to water, not water to acid — and (2) the reason involving the exothermic nature of the process and the danger of splashing/burns. Both are needed for full marks. This is a standard 2-mark question from the in-text questions of Section 2.2.1.