Pure and Impure Substances

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Pure and Impure Substances — Free MYP5 Chemistry Practice Questions

1QuestionCharacteristics of pure substances (fixed melting/boiling points)Concept Practice

2 marks~3 minCriterion A

The heating curve below shows a sample of pure water heated from

-10^\circ\text{C}

120^\circ\text{C}

. A flat plateau is observed at

0^\circ\text{C}

Identify what the plateau at

0^\circ\text{C}

represents, and state what this fixed temperature indicates about the purity of the sample. [1]

Explain why the temperature remains constant during this plateau, referring to energy transfer and particle behaviour. [1]

Solutions

2QuestionCharacteristics of pure substances (fixed melting/boiling points)Assessment Practice

2 marks~3 minCriterion D

A pharmaceutical company tests a batch of paracetamol for purity. Pure paracetamol has a melting point range of

169\,^{\circ}\mathrm{C}

171\,^{\circ}\mathrm{C}

Describe how the company could use melting point data to assess whether the batch is pure. [1]

The measured melting point of the batch is

163\,^{\circ}\mathrm{C}

. Evaluate one health risk this result poses to a patient taking the tablets. [1]

Solutions

3QuestionCharacteristics of pure substances (fixed melting/boiling points)Assessment Practice

3 marks~5 minCriterion C

A student heats a liquid sample and records its temperature every minute. The temperature rises steadily until it reaches

78^\circ\text{C}

, where it remains constant for several minutes before rising again.

Identify the phase change occurring at

78^\circ\text{C}

. [1]

Explain why a pure substance produces a plateau at a single fixed temperature rather than boiling over a temperature range. [1]

A second liquid sample boils between

78^\circ\text{C}

and

84^\circ\text{C}

with no clear plateau. Evaluate what this difference reveals about the purity of each sample. [1]

Solutions

4QuestionDefinition of mixtures and types (homogeneous vs heterogeneous)Assessment Practice

3 marks~5 minCriterion B

Four mixtures are analysed in a food-science laboratory. Their properties are recorded below.

Mixture — Salt water / Sand in water / Air / Oil and vinegar

Appearance	Clear / Cloudy / Clear / Two distinct layers
Number of phases	1 / 2 / 1 / 2

Particle size — Molecular / Visible grains / Molecular / Visible droplets

State the classification (homogeneous or heterogeneous) of each mixture. [1]

Deduce the relationship between the number of phases observed and the type of mixture. [1]

Evaluate whether the number of phases alone is a sufficient criterion for classifying a mixture, using evidence from the data above to support your reasoning. [1]

Solutions

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