SC25 Qualitatuive Analysis: Tests for Ions Flashcards
What is Qualitative Analysis?
Qualitative analysis is when the substance is classified or identified based on physical or chemical properties like solubility, chemical reactivity, molecular weight, melting point, mass spectra, etc.
What is Quantitative Analysis?
Quantitative analysis mentions the analyses in which the amount or concentration of an analyte may be expressed and determined (estimated) as the numerical value in the given appropriate units.
What does the Flame test, test for?
A flame test is carried out to test the presence of some
positive metal ions (cations).
Explain how to carry out a Flame Test.
- Clean a platinum/nichrome wire (wire loop) using hydrochloric acid to clean
any impurities - Pick up a small sample of the solid using the wire
- Use the wire to place the sample in the blue non-luminous part of the flame
- Observe and record the flame colour
What properties should the wire used for a flame test have?
Wire must be an inert (unreactive) metal like nichrome or platinum not give any colour. It must also have a high melting point
Give the list of Elements tested using the flame test and the flame colour each generates.
Calcium Orange/Red
Sodium Yellow
Potassium Lilac
Copper Green/Blue
Lithium Deep Red
Strontium Deep Red
Compared to simple laboratory tests such flame tests,
using scientific instruments may improve:
1.Sensitivity -they can detect much smaller amounts
2.Accuracy - they give values closer to the true values
3. Speed
How does Flame photometry work?
The flame photometer measures the light intensity of the
flame colours produced by metal ions. Its data is used to
determine the concentration of a metal ion in a dilute
solution.
Is Flame photometry Quantitative or Qualitative Analysis?
Quantitative
State what solution we use when we do precipitation reactions, and why we do them.
sodium hydroxide solution
Some metal ions do not burn with a
coloured flame so a flame test will not
identify them.
How does precipitation reaction work and how can we identify the cation preasent.0
- We can identify some cations by adding a A few drops of sodium hydroxide solution
(NaOH(aq)) to a solution of the ion. - If a precipitate forms on the addition of the NaOH, the colour of the precipitate can be
used to identify the cation
State all the Cations tested and the colour of precipitate formed after adding sodium hydroxide solution.
Aluminium white percipitate
Calcium white percipitate
copper(II) Pale bluepercipitate
iron(II) green percipitate
iron(III) brown (rust)percipitate
How do we distinguish between Aluminium and Calcium since they both produce a white precipitate
- Both the aluminium and calcium ions give a white precipitate when a few drops of sodium hydroxide are added.
- In order to distinguish between them, Excess sodium hydroxide is added
The aluminium hydroxide precipitate dissolves to form a colourless solution, whilst the calcium hydroxide precipitate remains.
State the Test for the Ammonium ion.
- The ammonium ion (NH4+) is a positive nonmetal ion used in fertilisers.
- To test its presence, sodium hydroxide is added.
- There is no precipitate formed.
- The mixture is then heated, which produces ammonia gas, which turns damp red litmus paper or damp universal indicator blue.
- This reaction can also be used as a test for hydroxide ions: addition of ammonium chloride will produce ammonia gas in the presence of hydroxide ions.
How do we test for Carbonate?
Add acid (e.g. HCl)
Result: - Effervescence-
- Add limewater—- turns cloudy/milky
How do we test for Sulfate?
Test: Add dilute HCl and then a few drops of
BaCl2 solution
Result: White precipitate formed
How do we test for Halide ions using Silver Nitrate?
- Make a solution of the halide
- acidify with dilute nitric acid – this prevents the precipitation of other salts
- add a few drops of AgNO3 solution
- Add to the precipitate dilute NH3 solution
- if a precipitate still exists, add concentrated ammonia solution
What effect does sunlight have on silver halides?
- If white AgCl is left in sunlight, it turns greyish in colour. This is because AgCl decomposes into silver and chlorine.
How do we differentiate the Hallides since the colour of the precipitate is very similar?
CHLORIDE white ppt of AgCl soluble in dilute ammonia
BROMID cream ppt of AgBr is insoluble in dilute ammonia
But soluble in concentrated.
IODIDE yellow ppt of AgI is insoluble in dilute and
concentrated ammonia
solution
State the Physical test for water.
Test: Boiling temperature/Melting temperature
Result: 100 oC/0 oC
State the Chemical test for water.
Test: add to white anhydrous copper(II)
sulfate
Result: turns blue
