Test For Ions

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You Can Use Chemical Tests to Identify Positive Ions

Positive ions (or cations) include things like the ions of the Group 2 metals and ammonium ions.

USE THE FLAME TEST TO IDENTIFY GROUP 2 IONS

Compounds of some Group 2 elements burn with characterisic colours. You can identify them using the fame test:

Dip a nichrome wire loop in concentrated hydrochloric acid.

Then dip the wire loop in the unknown compound

Hold the loop in the clear blue part of a Bunsen burner flame.

Observe the colour change:

Calcium, Ca2+ BRICK RED

Strontium, Sr2+ RED

Barium Ba2+ PALE GREEN

USE RED LITMUS PAPER AND NaOH TO TEST FOR AMMONIUM IONS

Ammonia gas is alkaline - so you can test for it using a damp piece of red litmus paper. The litmus paper needs to be damp so the ammonia gas can dissolve. If there;s ammonia present, the paper will turn blue.

If you add hydroxide ions to a solution containing ammonium ions they will react to produce ammonia gas and water.

You can use this reaction: NH4+(aq) + OH- ------> NH3(g) +H2O(l) to test whether a substance sontains ammonium ions. Add some dilute NaOH solution to your mystery substance in a test tube and gently heat the mixture. If there's ammonia given off, there must be ammonia ion present.

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You Can Use Chemical Tests To Identify Negative Ions Too

Negative ions (or anions) include things like halide ions, hydroxide ions, sulfate ions and carbonate ions.

TEST FOR SULFATES WITH HCl AND BARIUM CHLORIDE

Add a little dilute HCl followed by BaCl2. If a white precipitate is formed, it means the original compound contained a sulfate.

USE A pH INDICATOR TO TEST FOR HYDROXIDES

Hydroxide ions make solutions alkaline. So if you think a solution might contain hydroxide ions you can use a pH indicator to test it:

Dip a piece of red litmus paper into the solution.

If hydroxide ions are present the paper will turn blue.

TEST FOR HALIDES WITH SILVER NITRATE SOLUTION

To test for chloride, bromide or iodide ions just add dilute nitric acid followed by silver nitrate solution.

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.

HYDROCHLORIC ACID CAN HELP DETECT CARBONATES

You can test to see if a solution contains carbonate ions (CO3 2-) by adding an acid:

When you add dilute HCl, a solution containing carbonate ions will fizz. This is because the carbonate ions react with the H+ ion in th acid to give carbon dioxide.

CO3 2-(s) + 2H+(aq) ------> CO2(g) + H2)(l)

You can test for carbon dioxide using limewater.

Carbon dioxide turns limewater cloudy - just bubble through a test tube of lime water and watch what happens. If the lime water goes cloudy, your solution contained carbonate ions.