8.1.1 Pure Substances

• What is a pure substance in chemistry?

  A single element or compound not mixed with any other substance.
• Chemical analysis (also known as analytical chemistry) is about…?

  The instruments and methods used to separate, identify, and quantify different substances.
• Which of the following is an example of a pure substance? Clay, fruit juice, milk or distilled water?

  Distilled water.
• True or false? Pure substances always melt and boil at specific temperatures.

  True.
• What can be used to distinguish pure and impure substances?

  Melting and boiling point data.
• Pure water boils at 100°C, and freezes at 0°C. A student is given an unknown, transparent, colourless liquid. What two tests could be done to see if the liquid is pure water?

  • Boiling it to see if the boiling point is 100°C.
  • Freezing it to see if the freezing point is 0°C.
• A student tests the melting point of a sample of salol. The sample starts melting at 36°C, but it doesn't melt completely until the temperature is 48°C. Is the sample chemically pure, or impure?

  Impure.
• If we test a substance for purity by measuring its boiling point, is that a physical test or a chemical test?

  Physical.