Tests for Ions Sunday, 11 November 2018 L.O: To understand some ways in which you can test for negative and positive ions, and to be able to use the results of chemical tests to identify compounds.

Starter Solve the anagrams to fill in the gaps… Ionic bonds form when a metal reacts with a non-metal. Metals form positive ions; non-metals form negative ions. Ionic bonds are the strong electrostatic forces of attraction between oppositely charged ions. These ions are arranged in a regular way to form giant ionic lattices. Ionic compounds often form crystals as a result. etmla aemn-tnol eiptsvio etavinge littatreecosc caitartotn ppoyilsote rurgale tinag iattlsec muodpncso sasryclt

Testing Ionic Compounds Ionic compounds are made up of positive and negative ions. You might be asked to find out exactly which ions a compound contains. To do this there are different tests that you can carry out which identify whether specific ions are present.

Testing for Positive Metal Ions Flame Tests Compounds of some metals burn with a characteristic colour. You can test for various metal ions by carrying out a flame test.

Flame Tests Flame tests involve putting the substance in a flame and seeing what colour the flame goes. You need to know the colours of the flames produced by the following metal ions: Ion Chemical Formula Colour of Flame Lithium Li+ Crimson Sodium Na+ Yellow Potassium K+ Lilac Calcium Ca2+ Yellow-Red Barium Ba2+ Green

Match up! yellow lilac crimson yellow-red green barium lithium potassium sodium calcium

Doing a Flame Test To flame-test a compound in the lab: Dip a clean wire loop into a sample of the compound Put the wire loop in the clear blue part of the Bunsen flame (the hottest bit). Clean the wire loop by dipping it into hydrochloric acid. Rinse the wire loop in distilled water.

Testing for Positive Metal Ions Precipitation Reactions You can test for some metal ions using precipitation reactions.

Precipitation Reactions Many metal hydroxides are insoluble and precipitate out of solution when formed. Some of these hydroxides have characteristic colours, so you can test for some metal ions by adding a few drops of sodium hydroxide solution to a solution of the substance you’re testing. If you get a coloured insoluble hydroxide you can then tell which metal was in the compound.

Colours of Precipitates The colours of the precipitates formed by different metal ions are shown below. The results of adding NaOH(aq) to solutions of different metal ions.

Precipitation Reactions Watch the video to see some precipitation reactions. (skip through testing for ammonium ions)

Testing for Negative Metal Ions Carbonate Ions You can test a gas to see if it’s carbon dioxide by bubbling it through limewater

Testing for Carbonate Ions You can test a gas to see if it’s carbon dioxide by bubbling it through limewater. If carbon dioxide is present a white precipitate will form and the limewater will turn cloudy. When a carbonate reacts with a dilute acid you get a salt, water and carbon dioxide. This means you can use the test for carbon dioxide to test for carbonate ions (CO32-): if you react a substance with an acid and it gives off a gas that turns limewater cloudy, then the gas is carbon dioxide and the substance must contain carbonate ions.

Testing for Carbonate Ions Here is an image showing a test for carbonate ions:

Testing for Negative Metal Ions Halide Ions You can test for halide ions by adding dilute nitric acid followed by silver nitrate solution.

Testing for Halide Ions To test for chloride (Cl–), bromide (Br–) or iodide (I–) ions, add dilute nitric acid (HNO3), followed by silver nitrate solution (AgNO3). A chloride gives a white precipitate of silver chloride. A bromide gives a cream precipitate of silver bromide. An iodide gives a yellow precipitate of silver iodide. Here are the equations for these reactions:

Q: Which halide ions are contained in which precipitate? Iodide Chloride Bromide Q: Which halide ions are contained in which precipitate?

Which acid to use? It’s important to use dilute nitric acid for this test rather than hydrochloric acid. Hydrochloric acid would introduce chloride ions to the solution, so a white precipitate would be formed regardless of whether the solution originally contained chloride ions or not.

Testing for Negative Metal Ions Sulfate Ions You can test for sulfate ions by adding dilute hydrochloric acid followed by barium chloride solution.

Testing for Sulfate Ions To test for a sulfate ion (SO42–), add dilute hydrochloric acid (HCl), followed by barium chloride solution (BaCl2). A white precipitate of barium sulfate means the original compound was a sulfate. The ionic equation for this reaction is: H

Identifying Compounds You could be asked to identify a compound from the results of different tests. Just use the information that you’re given to work out what you’ve got. Example Question: Substance A is a white powder that is soluble in water. When a sample of substance A is held in a Bunsen flame, the flame burns with a green colour. When dilute nitric acid is added to a solution of substance A there is no reaction and no gas is given off, but if silver nitrate solution is then added a white precipitate is formed. Identify substance A.`

Questions What colour flames are produced when compounds containing the metal ions below are held in a Bunsen flame? a) Barium b) Potassium c) Lithium Give the names of the three metals that will form a white precipitate when sodium hydroxide solution is added to a solution containing a compound of the metal. Which one of these precipitates will redissolve when excess NaOH is added? What gas is produced when a carbonate reacts with a dilute acid? Describe how to test a solution to see if it contains bromide ions. What ions can be detected by adding dilute hydrochloric acid followed by barium chloride solution?

Answers a) Green b) Lilac c) Crimson Calcium, aluminium and magnesium. Aluminium (hydroxide). Carbon dioxide. Add dilute nitric and then silver nitrate solution. If a cream precipitate forms then the solution must contain bromide ions. Sulfate ions.

Exam Style Questions Substance X is a white powder that is an ionic compound. When a flame test is done on the compound, a red flame is produced. When dilute sulfuric acid is added to substance X, the gas that is produced turns limewater cloudy. Give the name of compound X. Substance Z is an ionic compound in solution. When sodium hydroxide solution is added to the solution containing substance Z, a blue precipitate is formed. When dilute hydrochloric acid is added followed by barium chloride solution, a white precipitate is formed. Name substance Z. Tom thinks he has a sample of iron(II) chloride solution. Describe how he could test the sample to see if it is iron(II) chloride and state what results he would expect to see.

Answers Calcium carbonate. The red flame in the flame test shows that there are calcium ions in the compound. The limewater turning cloudy shows that carbonate ions are present. Copper(II) sulfate. Copper ions react with sodium hydroxide to form a blue precipitate. The white precipitate formed after barium chloride and acid are added shows that there are sulfate ions. He should add sodium hydroxide solution to test for iron(II) ions. If the sample is iron(II) chloride solution a green precipitate will form. He should add dilute nitric acid followed by silver nitrate solution to test for chloride ions. If the sample is iron(II) chloride solution a white precipitate will form.

Homework Complete the RAG sheet AND answer the exam style questions… Due in: next lesson