Chemical Tests Flashcards
What is qualitative analysis
Qualitative analysis is the process of performing chemical tests on unknown compounds to determine their identity. These include tests for cations and anions
Describe the process of a flame test
Dip the tip of a platinum wire into concentrated hydrochloric acid and then in the flame of a bunsen burner in order to clean it and ensure that it doesn’t color the flame from previous tests. Once clean, the wire is dipped again in acid and used to pick up a few crystals of the compound to be tested. When the wire is held at the end of a blue, non-luminous flame, a new color in the flame will appear
What color does the lithium cation (Li+) produce
Red
What color does the sodium cation (Na+) produce
Orange
What color does the potassium cation (K+) produce
Lilac
What color does the calcium cation (Ca2+) produce
Orange red
What color does the copper cation (Cu2+) produce
Green
Why should the platinum wire be cleaned first before using it for the flame test
To avoid contamination and ensure that the wire won’t color the flame from previous tests
Why is a non-luminous flame used
A non-luminous flame is used to clearly see the color of the cation. The yellow color of the luminous flame would mask the color and visibility of the flame color of the test sample
What are the two possible tests for cations and which cations are they for
Flame test: Lithium, Sodium, Potassium, Calcium, Copper(II)
Adding sodium hydroxide solution: Copper(II), Iron(II), Iron(III)
Describe the process of adding sodium hydroxide to identify a cation
Add aqueous sodium hydroxide solution dropwise and then in excess to an aqueous solution containing a metal cation. This will produce a metal hydroxide. Since most metal hydroxides are insoluble, the color of the insoluble precipitate can be used to identify the cation
What color is the precipitate produced when adding sodium hydroxide to copper(II) cations
Blue
What color is the precipitate produced when adding sodium hydroxide to copper(II) cations
Blue
What color is the precipitate produced when adding sodium hydroxide to iron(II) cations
Green
What color is the precipitate produced when adding sodium hydroxide to iron(III) cations
Yellow/orange
Why can’t potassium ions be identified by adding sodium hydroxide
Potassium’s hydroxide is soluble
What is the test for ammonium cation
Add Sodium hydroxide and heat the mixture. Ammonia gas will form, which is colorless. However it can turn moist red litmus paper blue
What are the tests for anions and which anions are they for
Add solutions of nitric acid and silver nitrate: chloride ions, bromide ion, iodide ions (halides)
Add solutions of nitric acid or hydrochloric acid: Carbonate ions
Add solutions of hydrochloric acid and barium chloride: Sulfate ions
Describe the test for halides
Add dilute nitric acid in order to remove the carbonate ions present. Then add an aqueous solution of silver nitrate (AgNo3) and observe the precipitate form
Why is nitric acid added in the test for halides
The acid is added to remove carbonate ions. If carbonate ions were present, they would react with silver ions and form a white silver carbonate precipitate that would interfere with the results. Adding the acid dissolves the precipitate with effervescence
Why is Hydrochloric acid not used for the test instead of nitric acid
Hydrochloric acid also had chloride ions, and thus you cannot confirm that the formation of the white precipitate is because of their presence in the compound and not in the acid
What color is the precipitate produced when adding silver nitrate to chloride anions
White
What color is the precipitate produced when adding silver nitrate to bromide anions
Cream
What color is the precipitate produced when adding silver nitrate to iodide anions
Yellow
Describe the test for sulfate
Add aqueous barium chloride and dilute hydrochloric acid in the solution. A white precipitate should form
What color is the precipitate produced when adding barium chloride to sulfate anions
White
Describe the test for carbonate
Add aqueous nitric acid. Vigorous effervescnece is produced and a colorless gas. If the gas turn limewater cloudy, then the anion is carbonate
What is the test for ammonia
Hold damp red litmus paper to the mouth of a tube. The red litmus paper will turn blue
What is the test for carbon dioxide
Carbon dioxide is bubbled through limewater. The limewater should turn cloudy
What is the test for hydrogen
Hold a lighted splint near the mouth of a tube. A pop sound should be heard
What is the test for oxygen
Hold a glowing splint near the mouth of a tube. Oxygen should relight it
What is the test for chlorine
Hold damp litmus paper near the mouth of a tube. It should bleach the paper
