Chemical Analysis Tests

Question 1

What does a flame test do?

Answer 1

show the presence of certain metal ions (cations) in a compound

Question 2

What is a cation?

Answer 2

metal ions that are positively charged

Question 3

How do you carry out a flame test?

Answer 3

-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

Question 4

Why do we use platinum or nichrome wire?

Answer 4

they have a high melting point and are inert

Question 5

What is the flame colour for lithium?

Answer 5

red

Question 6

What is the flame colour for potassium?

Answer 6

lilac

Question 7

What is the flame colour for copper (II)?

Answer 7

blue-green

Question 8

What is the flame colour for sodium?

Answer 8

yellow

Question 9

What is the flame colour for calcium?

Answer 9

orange-red

Question 9

How do you test for cations Cu +2, Fe +2 and Fe +3?

Answer 9

-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

Question 10

What is the test for the cation NH +4?

Answer 10

-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

Question 11

When testing for copper (II) ions (CU +2), what type of precipitate forms?

Answer 11

blue gelatinous precipitate of CU(OH)2

Question 12

When testing for iron (II) ions (Fe +2), what type of precipitate forms?

Answer 12

green gelatinous precipitate of Fe(OH)2

Question 13

When testing for iron (III) ions (Fe +3), what type of precipitate forms?

Answer 13

red/brown gelatinous precipitate of Fe(OH)3

Question 14

How would you test for the anion SO4 -2?

Answer 14

-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

Question 15

How would you test for the anion CO3-?

Answer 15

-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

Question 16

What is the test for the anions Cl-, Br- and I-?

Answer 16

-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)

Question 17

What colour precipitate does chloride ions (Cl-) form when reacting with silver nitrate?

Answer 17

white precipitate

Question 18

What colour precipitate does bromide ions (Br-) form when reacting with silver nitrate?

Answer 18

cream precipitate

Question 19

What colour precipitate does iodide ions (I-) form when reacting with silver nitrate?

Answer 19

yellow precipitate

Question 20

How would you test for the presence of water using anhydrous copper (II) sulfate?

Answer 20

-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

Question 21

What is the physical test for pure water?

Answer 21

-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