Required Practical: Identifying Ions

Aim

To identify the ions present in unknown salts using a combination of chemical tests, and to determine the identity of the salts.

Materials

  • Samples of unknown salts and salt solutions
  • Dilute hydrochloric acid (HCl)
  • Dilute nitric acid (HNO3)
  • Dilute sodium hydroxide (NaOH)
  • Dilute barium chloride (BaCl2) solution
  • Dilute silver nitrate (AgNO3) solution
  • Limewater
  • Bunsen burner
  • Nichrome wire and needles with a handle
  • Test tubes
  • Test tube holder
  • Teat pipette
  • Safety goggles

Method

There are many tests you can use to identify the ions in the unknown salts. These are the tests we will use in this practical:

Flame test:

a. Dip the nichrome wire into the unknown salt, and hold the wire in a Bunsen burner flame

b. Observe the colour of the flame

c. Record the observations

Hydroxide precipitate test:

a. Add a few drops of sodium hydroxide (NaOH) to the unknown salt solution.

b. If a precipitate forms, add an excess of NaOH

c. Record the colour and nature of the precipitate

Carbonate ion test:

a. Add a few drops of dilute hydrochloric acid (HCl) to the unknown salt solution

b. Observe the reaction

c. Bubble the gas produced through limewater

d. Record the observations

Halide ion test:

a. Add a few drops of dilute nitric acid (HNO3) to the unknown salt solution

b. Add a few drops of dilute silver nitrate (AgNO3) solution to the same solution

c. Observe the colour and nature of the precipitate

Sulfate ion test:

a. Add a few drops of dilute HCl to the unknown salt solution

b. Add a few drops of dilute barium chloride (BaCl2) solution

c. Observe the colour and nature of the precipitate

Results

Record your results in a table. For example, the table below shows observations for three unknown salts.

SaltTestObservationInference
AFlame testOrange-red flameMust contain Ca²⁺
AHalide testWhite precipitateMust contain Cl⁻
BHydroxide precipitate testWhite precipitate that dissolves in excess NaOH to form a colourless solutionMust contain Al³⁺
BSulfate testMilky white precipitateMust contain SO4²⁻
CFlame testBlue-green flameMust contain Cu²⁺
CCarbonate testEffervescence, limewater turns cloudyMust contain CO3²⁻

The inference column shows what the observations mean. In this table, we’ve only included the positive results but you can also show the negative results. For example: Halide test – No precipitate – Must not contain a halide ion.

Conclusion

Each test provides valuable information about the specific ions present, and by combining the results, we can name the salt. It is important to balance the charges on the ions in the formula of the salt.

This involves determining the number of each ion in the salt and adjusting the subscripts to balance the charges.

For example, in the results table provided:

  • Salt A is identified as calcium chloride (CaCl2). The flame test was positive for Ca²⁺ ions, which requires an anion with a -2 charge or two anions with a -1 charge to balance. One calcium ion (Ca²⁺) requires two chloride ions (Cl⁻) to balance the charges.
  • Salt B is identified as aluminium sulfate (Al2(SO4)3). The hydroxide precipitate test and the sulfate test confirmed that the unknown salt contains aluminium ions and sulfate ions. To balance the charges, two aluminium ions (Al³⁺) are needed for every three sulfate ions (SO4²⁻).
  • Salt C is identified as copper carbonate (CuCO3). The flame test and carbonate test confirmed that the unknown salt contains copper ions and carbonate ions. One copper ion (Cu²⁺) requires one carbonate ion (CO3²⁻) to balance the charges.

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