Elements, Compounds and Mixtures: Answers

 
 

Is water classed as an element, compound or a mixture?

Water is a compound because it contains two different atoms (hydrogen and oxygen) chemically bonded together.

Describe how to produce pure, dry crystals of copper sulfate from a copper sulfate solution.

Gently heat the copper sulfate solution to evaporate the solvent. The remaining solution is cooled to allow the crystals to form. You would then need to filter off any remaining solution, then wash and dry the crystals.

What method would be used to separate the following:

  • pure water from seawater

  • ethanol from a mixture of ethanol and water

  • calcium carbonate from a mixture of calcium carbonate and water

To separate pure water from seawater, we can use simple distillation. This is because water has a much lower boiling point than salt (sodium chloride) since water is a simple covalent molecule while sodium chloride is giant ionic. Heating the seawater in a round bottom flask will cause the water to evaporate before the salt. The water vapour would make its way into the condenser, condense into liquid water and can be collected. The salt that is left remains in the round-bottom flask.

To separate ethanol and water, we use fractional distillation. It works ethanol and water have different boiling points. They are passed into a fractionating column which is hot at the bottom and cool at the top. They rise up the column and when the temperature matches their boiling point, they will condense and be collected. Since they condense at different temperatures, they can be collected separately.

To separate calcium carbonate from a mixture of calcium carbonate and water, you use filtration. Here you need to know that calcium carbonates, like most carbonates, are insoluble so it will not dissolve in the water. Remember that filtration is the method used to separate solids and liquids by passing the mixture through a funnel lined with filter paper.

To summarise:

  • Use simple distillation when you want to separate the solvent from the solute - i.e. water and salt in seawater

  • Use fractional distillation to separate two or more liquids mixed together - i.e. liquid ethanol and liquid water

  • Use filtration to separate a liquid and a solid - i.e. calcium carbonate and water, or sand and water

Why is a line drawn in pencil rather than an ink pen when carrying out paper chromatography?

If you were to draw a line in pen, the pen ink would run up the filter paper along with the solvent, making your chromatography paper all messy and making it difficult for you to analyse your results.