A definite melting point is more clearly found in which substance?

Correct answer: Pure substance

Direct answer: Option A, pure substance. A pure substance contains only one kind of particle, so its particles have a definite composition and a definite change of state. On heating, it melts at one characteristic temperature, provided the pressure is fixed. Step by step: ice made of pure water melts at a fixed temperature; impurities disturb the regular arrangement and usually make melting occur over a range. A is correct because purity gives a sharp melting point. B, soil, contains sand, minerals, water and organic matter, so it is a mixture and has no single sharp melting point. C, air, contains several gases, so it is a mixture. D, lemonade contains water, sugar, acids and other substances, so it also melts over a range. Melting point can therefore help test purity, although pressure and the substance itself must also be considered. Memory cue: pure substance gives a sharp point; mixture usually gives a melting range.