How to Determine if a Substance is Pure


A pure substance consists of only one type of substance not mixed with others. In contrast, a mixture is a substance which has two or more different substances mixed together (not chemically combined). In this chapter, we would look into how to differentiate if a substance is pure (or is it a mixture). Determining if a substance is pure is important because, for example, if a medication consists of impurities, it may cause harm when ingested.

To determine if a substance is pure in school laboratories, we can check the substance’s melting or boiling points or use chromatography (see Separation Techniques).

Solids

A pure solid has a constant/fixed melting point. ie. it will melt completely at one temperature only.

With impurities, the melting point of a substance is affected in two ways:

  1. The melting point is lowered. For example, pure ice melts completely at zero degree Celsius but the ice we make at home from tap water will start to melt before zero degree Celsius. The more impurities a substance contains, the lower its melting point will be.
  2. Melting would occur over a range of temperatures and not at a constant/fixed temperature.

Liquids

A pure liquid has a constant/fixed boiling point.

With impurities, the boiling point of a substance is affected in two ways too:

  1. The boiling point is increased. The more impurities a substance contains, the higher its boiling point will be.
  2. The substance would boil over a range of temperatures.

Pressure also affects boiling point of a substance. The greater the pressure exerted on the liquid, the higher its boiling point would be. Conversely, the lower the pressure exerted on the liquid, the lower its boiling point would be.

Examples

An impure sample of Z melts over a range of temperatures ranging between 117°C and 121°C. What can be said about a pure sample of Z’s melting point?

A: It would have a melting point of between 117°C and 121°C.
B: It would melt at temperatures lower than 117°C.
C: It would melt at temperatures higher than 121°C.
D: Its melting point cannot be determined.

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Answer is C. A pure sample would only melt at a fixed temperature and not over a range of temperatures and hence, A is wrong. Although D is also correct, the most suitable answer would be C as we know that impurities lower the melting point of a substance and hence, the melting point of a pure sample of Z would be higher than that of its impure sample.

 

Hikers A and B are boiling water simultaneously at the base of a mountain and on the top of a mountain respectively. Hiker B’s pot of water starts boiling much earlier than Hiker A’s. Why is this so?

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Answer: The atmospheric pressure at the top of a mountain where Hiker B is, is lower than at sea level, where Hiker A is. At lower atmospheric pressures, there is lesser pressure exerted on the water and hence, the boiling point of the water is lowered. Hence, the water boils faster than at the base of the mountain.

 


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