CIE Chemistry (US-0439): Specimen Questions with Answers 4 - 5 of 240

Question number: 4

Short Answer Question▾

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Pure water is neutral. Which one of these pH values is neutral? Put a ring around the correct answer.

Explanation

The pH of pure water is. In general, water with a pH lower than is considered acidic and with a pH greater than is considered basic.

Question number: 5

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Inks are mixtures of different dyes.

A student used paper chromatography to separate the dyes in a particular ink.

Describe how paper chromatography is carried out. You may draw a diagram to help explain your answer.

In your description include

  • the apparatus you would use,

  • How chromatography is carried out.

Explanation

You probably used paper chromatography as one of the first things you ever did in chemistry to separate out mixtures of coloured dyes - for example, the dyes which make up a particular ink. That’s an easy example to take, so let’s start from there.

Suppose you have three blue pens and you want to find out which one was used to write a message. Samples of each ink are spotted on to a pencil line drawn on a sheet of chromatography paper. Some of the ink from the message is dissolved in the minimum possible amount of a suitable solvent, and that is also spotted onto the same line. In the diagram, the pens are labelled , and the message ink as M.

Note: The chromatography paper will in fact be pure white - not pale grey. I’m forced to show it as off-white because of the way I construct the diagrams. Anything I draw as pure white allows the background colour of the page to show through.

The paper is suspended in a container with a shallow layer of a suitable solvent or mixture of solvents in it. It is important that the solvent level is below the line with the spots on it. The next diagram doesn’t show details of how the paper is suspended because there are too many possible ways of doing it and it clutters the diagram. Sometimes the paper is just coiled into a loose cylinder and fastened with paper clips top and bottom. The cylinder then just stands in the bottom of the container.

The reason for covering the container is to make sure that the atmosphere in the beaker is saturated with solvent vapour. Saturating the atmosphere in the beaker with vapour stops the solvent from evaporating as it rises up the paper.

As the solvent slowly travels up the paper, the different components of the ink mixtures travel at different rates and the mixtures are separated into different coloured spots.

The diagram shows what the plate might look like after the solvent has moved almost to the top.

It is fairly easy to see from the final chromatogram that the pen that wrote the message contained the same dyes as pen 2. You can also see that pen contains a mixture of two different blue dyes - one of which might be the same as the single dye in pen .