Chemical Analysis Tests Flashcards
What does a flame test do?
show the presence of certain metal ions (cations) in a compound
What is a cation?
metal ions that are positively charged
How do you carry out a flame test?
-dip platinum or nichrome wire in concentrated HCl to remove any impurities
-dip the wire into the salt being tested so some of it sticks
-hold the wire and salt in a non-luminous (roaring) bunsen burner flame
-observe the colour
Why do we use platinum or nichrome wire?
they have a high melting point and are inert
What is the flame colour for lithium?
red
What is the flame colour for potassium?
lilac
What is the flame colour for copper (II)?
blue-green
What is the flame colour for sodium?
yellow
What is the flame colour for calcium?
orange-red
How do you test for cations Cu +2, Fe +2 and Fe +3?
-place 1cm3 of the salt solution to be tested in the test tube
-slowly add drops of 0.5mol dm-3 sodium hydroxide until you observe a change
-continue to add more sodium hydroxide and record your observations
What is the test for the cation NH +4?
-add 2cm3 of sodium hydroxide solution to 1cm3 of the solution in a test tube
-place test tube in beaker of warm water and if ammonium ions are present then ammonia has (NH3 will form)
-the gas will turn damp red litmus to blue
When testing for copper (II) ions (CU +2), what type of precipitate forms?
blue gelatinous precipitate of CU(OH)2
When testing for iron (II) ions (Fe +2), what type of precipitate forms?
green gelatinous precipitate of Fe(OH)2
When testing for iron (III) ions (Fe +3), what type of precipitate forms?
red/brown gelatinous precipitate of Fe(OH)3
How would you test for the anion SO4 -2?
-add 1cm3 of 1 mol dm-3 HCl to 1cm3 of the unknown solution
-at this stage is effervescence is seen it is a carbonate
-add a few drops of barium chloride solution (BaCl2)
-if sulfate ions (SO4 -2) are present, a white precipitate of barium sulfate (BaSO4) will form
How would you test for the anion CO3-?
-add 1cm3 of 1 mol dm-3 HCl to 1cm3 of unknown solution
-if carbonate ions (CO3 -2) are present then effervescence will occur
-to be even more sure the gas can be tested for CO2 using limewater
What is the test for the anions Cl-, Br- and I-?
-add 1cm3 of 0.5 mol dm-3 nitric acid (HNO3) to remove impurities from the sample of potassium (chloride, bromide or iodide)
-add a few drops of silver nitrate solution (this reaction forms the silver salt)
What colour precipitate does chloride ions (Cl-) form when reacting with silver nitrate?
white precipitate
What colour precipitate does bromide ions (Br-) form when reacting with silver nitrate?
cream precipitate
What colour precipitate does iodide ions (I-) form when reacting with silver nitrate?
yellow precipitate
How would you test for the presence of water using anhydrous copper (II) sulfate?
-add anhydrous copper (II) sulfate (CuSO4) to the sample
-if water is present then the anhydrous copper (II) sulfate will change from white to blue
-this results in hydrated copper (II) sulfate
What is the physical test for pure water?
-check its boiling point
-gently boil it
-using a thermometer, check if its boiling point is exactly 100 degrees celsius
-if any impurities are present, they tend to raise the boiling point and depress the melting point of the pure substance
