8.3 - Qualitative analysis

Question 1

What is qualitative analysis?

Answer 1

Uses simple observations rather than measurements.

Question 2

How would you test for carbonates?

Answer 2

Carbonates react with acids to form CO2.

1) Add dilute HNO3 to the sample.
2) If you see bubbles, the unknown compound could be a carbonate.
3) Test for CO2 by bubbling through limewater. If CO2 present, reacts with limewater to form white precipitate of CaCO3 (s), which turns limewater cloudy.

Question 3

How would you test for sulfates?

Answer 3

Most sulfates are soluble in water, but BaSO4 is very insoluble.

Use aqueous barium nitrate/chloride (don’t use chloride if planning a halide test afterwards).
Ba+ ions react with SO4 2- ions to form BaSO4, which forms a white precipitate.

Question 4

How would you test for halide ions?

Answer 4

Aqueous silver ions react with aqueous halide ions to form a precipitate of silver halides.

Silver chloride = white
Silver bromide = cream
Silver iodide = yellow

Precipitates can be added to aqueous ammonia.
Silver chloride = soluble in dilute NH3
Silver bromide = soluble in conc NH3
Silver iodide = insoluble in conc NH3