61. In an experiment to determine the percentage of water in a solid hydrate by heating, what is the
best indication that all the water has been removed?
(A) The solid melts.
(B) The solid changes color.
(C) Water vapor no longer appears.
(D) Successive weighings give the same mass.
The correct answer is D. The water in a hydrate can be removed by heating. As the water is removed, the mass of the sample will decrease. If successive weighings give the same mass, this means all the water has gone. Depending on what size of your sample is, you may not be able to observe water vapour. As a result, C is not correct even though it looks reasonable. Some of the solid will change colour while some do not, B is not always true. A is not correct because it is not related to water removing. When the solid melt, it is at its melting point and that is all you know.
Following is question 62 of the series:
62. Which solid reacts with dilute hydrochloric acid at 25 °C to produce a gas that is more dense than air?
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