Unit 3 - Lesson 11: Tests for Cations & Anions Flashcards
What are cations?
Positive metal ions
What are anions?
Negative non-metal ions
What two tests can be used to identify cations in an ionic compound?
- The Flame Test
- Adding a Solution of Sodium Hydroxide to Form a Coloured Precipitate
Explain the method of ‘The Flame Test’.
- To clean a platinum/nichrome wire loop, dip it in dilute HCl.
- Hold the loop in a blue Bunsen Burner flame (hottest part).
- Once it burns without any colour, remove it from the flame and dip it into the sample you want to test.
- Place the loop with sample back in the blue part of the Bunsen Burner flame. Observe the colour.
What colour flame will be produced if the cation is Lithium?
Crimson-Red
What colour flame will be produced if the cation is Sodium?
Yellow-Orange
What colour flame will be produced if the cation is Potassium?
Lilac
What colour flame will be produced if the cation is Calcium?
Orange-Red
What colour flame will be produced if the cation is Copper (II)?
Blue-Green
Explain the method of ‘Adding a Solution of Sodium Hydroxide to Form a Coloured Precipitate’.
- Add a few drops of sodium hydroxide solution to a solution of the compound you want to test.
- If an insoluble hydroxide precipitate (solid) forms, the colour identifies the ion present.
(Relating to Q5) What colour will the iron (II) hydroxide precipitate of iron (II) be?
Iron (II) hydroxide precipitate is sludgy green.
(Relating to Q5) What colour will the iron (III) hydroxide precipitate of iron (III) be?
Iron (III) hydroxide precipitate is reddish brown.
(Relating to Q5) What colour will the copper (II) hydroxide precipitate of copper (II) be?
Copper (II) hydroxide precipitate is blue.
What is a precipitate?
A solid produced.
What does reacting a metal ion with a hydroxide ion produce?
Give an example.
A metal hydroxide.
Cu^2+ + 2OH^- → Cu(OH)2
(The ^2 represents a charge of positive two, meaning it is giving away two electrons; the ^- represents a charge of negative one, meaning it is gaining one electron.)
What colour can iron (II) hydroxide change over time and why?
Iron (II) hydroxide can change from a sludgy green to orange-brown at the top because it is left to oxidise to iron (III) hydroxide.
What method is used to test for ammonium ions (NH^4+)?
- Add sodium hydroxide solution to the unknown salt solution and warm it over a Bunsen Burner.
- Dampen a piece of red litmus paper and hold it in the mouth of the test tube.
- If ammonia is present, the paper will turn blue (showing it is now alkaline).
Level 8/9 Problem Solving - Strontium’s colour is similar to lithium. How could you know which substance you have?
Compare the flame of the unknown substance to the flame of a known compound of lithium or strontium.
What do carbonates produce?
CO2 (Carbon Dioxide)
What method is used to test for carbonates in an unknown substance?
- Add dilute HCl (acid) to the sample you wish to test.
- Pass the gas produced through limewater.
- If the limewater turns cloudy, the solid contains a carbonate.
https://i.pinimg.com/736x/ab/0a/e6/ab0ae62ff55729fb1d378886b05fee00.jpg
What chemical equation represents this method/reaction?
CO3^2- (s) + acid → CO2 (g) + H2O (l)
(The ^2- represents a charge of negative two)
What method is used to test for sulfates in an unknown substance?
- Add dilute HCl to the test sample. This removes traces of other ions (so to not confuse the results).
- Add barium chloride (BaCl2) solution.
- If a white precipitate (solid) is formed, the test sample included a sulfate. The precipitate is BaSO4.
What chemical equation represents this method/reaction?
Ba^2+ (aq) + SO4^2- (aq) → BaSO4 (s)
(^2+ represents a charge of positive two and ^2- represents a charge of negative two.)
What do sulfate ions (SO4^2-) produce when mixed with BaCl2 (barium chloride)?
A white precipitate.
What are halides?
Halogen (group 7) ions.
What method is used to test for the halides:
- Chloride (Cl^-)
- Bromide (Br^-)
- Iodide (I^-)
in an unknown substance?
- Add nitric acid (HNO3) to the test sample to remove traces of other ions (to not confuse the results).
- Add silver nitrate solution (AgNO3) to the sample.
- Observe the colour of any precipitate formed.
What colour precipitate of silver chloride does chloride produce?
White
Ag^+ (aq) + Cl^- (aq) → AgCl (s)
What colour precipitate of silver bromide does bromide produce?
Cream
Ag^+ (aq) + Br^- (aq) → AgBr (s)
What colour precipitate of silver iodide does iodide produce?
Yellow
Ag^+ (aq) + I^- (aq) → AgI (s)
Tests for Anions Summary (Pinterest Picture)
https://i.pinimg.com/736x/a9/ec/64/a9ec644387d44823153a95cfc48af782.jpg
Why is the type of acid used in these tests important?
If I add dilute hydrochloric acid to test for chloride, we wouldn’t know if the test was positive because there is chloride from the salt or the HCl.
If I added dilute sulfuric acid to test for sulfate, I wouldn’t know if the test was positive because there’s sulfate from the salt or H2SO4.
