5.5 Tests for Anions

Question 1

What do “qualitative tests” mean when referring to testing for ions?

Answer 1

Measuring the colour of a precipitate

Question 2

What do “quantitative tests” mean when referring to testing for ions?

Answer 2

Titrations, which measure numerical data and let you work out the amount of a substance in a sample

Question 3

How should you choose your samples when testing water in an area?

Answer 3

You should take samples in slightly different places and test them all.

Question 4

How should you choose your samples when testing a chemical that’s been made in industry?

Answer 4

You should take a number of samples at random from each batch to test.

Question 5

When testing the purity of a substance you’ve crystallised by testing its melting point, why should you take a couple samples?

Answer 5

In case there’s some variation across the substances.

Question 6

How do you test for carbonates? (3)

Answer 6

• To test for carbonate ions in solution, add some dilute acid.
• If there are carbonate ions present, the mixture will fizz - this is because the carbonate will react with the acid to produce carbon dioxide gas.
• You can check to see if a gas is carbon dioxide by bubbling it through limewater. If it is carbon dioxide, the limewater turns cloudy.

Question 7

How do you test for sulfates? (2)

Answer 7

• To test for sulfate ions in solution, first add some dilute hydrochloric acid to the test sample - this stops any precipitation reactions not involving sulfate ions from taking place.
• Then add some barium chloride or barium nitrate solution. If there are sulfate ions in the solution, a white precipitate of barium sulfate will form.

Question 8

How do you test for halide ions?

Answer 8

To test for chloride ions, bromide ions or iodide ions, add some dilute nitric acid to get rid of carbonate and sulfate impurities, followed by a few drops of silver nitrate solution. The silver ions from the silver nitrate react with the halide ions to form a precipitate.

Question 9

What does a positive test for chloride look like?

Answer 9

A chloride gives a white precipitate of silver chloride.

Question 10

What does a positive test for bromide look like?

Answer 10

A bromide gives a cream precipitate of silver bromide.

Question 11

What does a positive test for iodide look like?

Answer 11

An iodide gives a yellow precipitate of silver iodide.

Question 12

What does the term ‘anion’ mean?

Answer 12

A negatively charged ion

Question 13

A chemical reaction is taking place in a test tube. What methods could be used to collect the gas?

Answer 13

• Attach the top of the test tube to a gas syringe.
• Place an upside down measuring cylinder over the test tube to trap the gas.

Question 14

When testing for sulfate ions, why do we have to add hydrochloric acid first?

Answer 14

To react with any carbonate ion impurities