What are the key points in recognising if a substance is pure?

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Sheena Nguyen

I have worked with children from a wide range of age groups and backgrounds and have done private tutoring previously for KS3 English students.

Purity can be defined as the absence of any impurities -- or types of matter other than the substance itself. The physical properties of a substance can be used to establish its purity. These properties include the melting point and boiling point. Different substances tend to have different melting and boiling points, and any pure substance will have a specific melting and boiling point. However, the presence of impurities will cause a lower melting point as well as a change in boiling point. The most accurate means of determining the purity of a substance is through the use of analytical methods. These methods, widely used in different industries, mostly involve chemical analysis, which can pinpoint the presence, identity and amount of impurities in the sample. The most simple chemical methods include gravimetry and titration. There are also the more advanced light-based or spectroscopic methods, such as UV-VIS spectroscopy, nuclear magnetic resonance and infrared spectroscopy. Chromatographic methods, such as gas chromatography and liquid chromatography, can also be used. Other methods used in testing the purity include mass spectroscopy, capillary electrophoresis, optical rotation and particle size analysis.

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