Chemical tests to identify anions

Connector –

1. What is an anion?

2. What happens when potassium nitrate is heated?

3. What gas will turn moist red litmus paper blue?

4. What is the chemical test for water vapour?

An anion is an atom or group of atoms with a negative charge.

Experiment – Testing for anions.1

1. Testing for hydroxides

Use a glass rod to put a spot of sodium hydroxide on red litmus paper and on universal indicator paper. Repeat using ammonia soln.

2. Testing for carbonates and sulphites

a. Put a sample of sodium carbonate in a boiling tube, then carefully add an equal dilute HCl. Pour the invisible gas from the boiling tube into another boiling tube that contains 2cm3 of limewater and gently shake.

b. Repeat the experiment using sodium sulphite, but this time test the gas using

i. a piece of filter paper soaked in acidified Na2Cr2O7;

ii. moist blue litmus paper, over the mouth of the boiling tube.

Write word and balanced symbol equations for the two decomposition reactions.

Carry out the following tests and record the test and your observations:

Hydroxides OH-

Carbonates, sulphites

Write the balanced symbol equation

Write the balanced symbol equation

Carbonates

Sulphite ion; SO32-

Questions

Experiment – Testing for anions.2

Carry out the following tests and record the test and your observations:

3. Testing for sulphatesPut 2cm3 of sodium sulphate in a test tube add the same volume of

dilute HCl, and barium chloride soln.

4. Testing for halide ionsIn separate test tubes put 2cm3 of potassium chloride, potassium

bromide and potassium iodide soln., i. to each one add 2cm3 of dilute nitric acid followed by 2cm3 of

silver nitrate soln. – record your observationsii. Now add 2cm3 of dil. ammonia soln.iii. Then add 2cm3 of conc. ammonia soln.

Write word and balanced symbol equations for the reactions that occurred in the precipitation reactions.

(Note: Ignore the acids in tests 3 & 4.)

Sulphate

Write the balanced symbol equation

Halide ions

Write the balanced symbol equation

Silver nitrate + potassium bromide silver bromide + sodium nitrate

CREAM

Silver nitrate + potassium iodide silver iodide + sodium nitrate

YELLOW

Halide ions

Summary

• Litmus paper turns __________ if hydroxide ions are present.• If you add __________ acid to sulphite ions the __________ gas

produced turns litmus paper __________ .• If you add hydrochloric acid to carbonate ions the __________ gas

produced turns __________ milky.• Barium chloride solution produces a white __________ with sulphite

and sulphate ions but only the __________ ion precipitate will dissolve in hydrochloric acid.

• Adding acidified __________ nitrate solution to halides forms a precipitate that is white with chloride ions, __________ with bromide ions and yellow with __________ ions.

blue carbon dioxide cream hydrochloric

iodide limewater precipitate red silver

sulphite sulphur dioxide

Testing for anions 1Dil. HClSodium hydroxide soln..Solid Sodium carbonate Solid Sodium sulphite limewaterRed litmus paperBlue litmus paperSolution of acidified potassium (or sodium) dichromateStrips of filter paper (to be dipped into acidified K2Cr2O7)UI paperTest tubesBoiling tubesTest tube racks

Testing for anions 2Dil. HClDil. HNO3Sodium sulphate solnNaCl soln.NaBr solnNaI solnBarium chloride soln.Silver nitrate solnDilute Ammonia soln.Concentrated ammonia solnTest tubesBoiling tubesTest tube racks