Question 2 — Qualitative analysis of two unknown solids
Run the standard test series on solids E and F, record every observation, and name the ions present.
RUN IT ON THE BENCH
Work through it, then fill in your answers below.
ON YOUR BENCH — FROM THE CONFIDENTIAL INSTRUCTIONS
Question 2 — solids
- solid E(1 g)
- solid F(1 g)
Question 2 — reagents
- aqueous sodium hydroxide
- aqueous ammonia
- dilute nitric acid
- aqueous silver nitrate
- aqueous barium nitrate
- acidified aqueous potassium manganate(VII)
- limewater and apparatus to test for carbon dioxide
- red and blue litmus papers
- wooden splints
Question 2 — apparatus
- rack of 3 test-tubes(1)
- boiling tubes(3)
- apparatus to carry out a flame test
- Bunsen burner and means to light it
- heat-proof mat
- test-tube holder(1)
- dropping pipettes
- access to water and distilled water
THE BENCH — TWO UNKNOWN SOLIDS, DISSOLVED AND SPLIT INTO PORTIONS
Solid E has been dissolved in distilled water and divided into portions. Pick a test — you will see exactly what a candidate at the bench would see, and nothing more.
Why nitric acid before barium or silver nitrate?
Carbonate gives a white precipitate with barium nitrate, and so does sulfate — so without the acid you cannot tell them apart. Nitric acid destroys any carbonate first, fizzing it off as carbon dioxide. Whatever precipitate survives is the real answer. It has to be nitric acid, because hydrochloric would add chloride to a sample you are about to test for halides.
Why does 'in excess' get its own mark?
Several cations give a white precipitate with a little sodium hydroxide. What separates them is what happens next. Zinc and aluminium both redissolve in excess alkali, but only zinc redissolves in excess ammonia as well. So the sequence — precipitate, then dissolve — is two observations and two marks, and reporting only the first loses half.
Why is 'no change' worth writing down?
Because it is evidence. Cold sodium hydroxide doing nothing to solid E rules out every cation that would have precipitated, which is what makes the ammonia on warming so conclusive. Examiners credit “no change”; they cannot credit a blank.
YOUR TASKS · 0/4
- ✓Run every test on solid E
- ✓Run every test on solid F
- ✓Identify the ions in solid E
- ✓Identify all three ions in solid F
IDENTIFICATION
Work through every test on a solid before naming its ions. Guessing early is how you lose the marks you already earned by observing carefully.
YOUR ANSWERS
To the first portion of solution E, add about 2 cm depth of aqueous sodium hydroxide. Record your observations.
Gently warm the product from 2(a)(i). Test any gas produced. Record your observations.
To the second portion of solution E, add about 1 cm depth of dilute nitric acid followed by a few drops of aqueous barium nitrate. Record your observations.
To the third portion of solution E, add a few drops of acidified aqueous potassium manganate(VII). Record your observations.
Identify solid E.
Carry out a flame test on solid F. Record your observations.
To the first portion of solution F, add about 1 cm depth of dilute nitric acid followed by a few drops of aqueous silver nitrate. Record your observations.
To the second portion of solution F, add aqueous sodium hydroxide dropwise and then in excess. Record your observations.
To the third portion of solution F, add aqueous ammonia dropwise and then in excess. Record your observations.
Identify the three ions in solid F.