Characteristic tests

Level 1

In chemistry, many substances are invisible to the naked eye: some gases are colorless and odorless, some solutions are transparent… To recognize them, chemists use characteristic tests. A characteristic test is a simple experiment that gives a precise result (color change, appearance of a precipitate, specific reaction) when a given chemical species is present.

Identifying water

To know if a liquid contains water, we use anhydrous copper sulfate. It is a white solid that turns blue on contact with water.

  Example:
Anhydrous copper sulfate is white. 
It is added to a solution. 
If it turns blue, there is water in this solution.

Identifying carbon dioxide

Carbon dioxide ($CO_2$) is a colorless gas. To test for it, limewater, which is colorless, is used. Upon contact with $CO_2$, limewater turns cloudy (formation of a white precipitate).

Note: A precipitate is a solid suspended in a liquid, formed by a chemical reaction

  Example 
By blowing through a straw into limewater, the exhaled $CO_2$ turns the liquid whitish.

Identifying dioxygen ($O_2$).

Dioxygen is an oxidising gas (it supports combustion). Plunge a glowing match into the gas to be tested: if it relights brightly, dioxygen is present.

  Example 
In a tube filled with dioxygen, a glowing match reignites brightly.

Identifying dihydrogen ($H_2$)

Dihydrogen is a flammable gas. Bring a flame near the opening of the tube containing the gas: a detonation ("pop") occurs if it is dihydrogen.

  Example 
Pour an acid onto zinc, a gas is released. 
Bringing a lit match close causes a small "pop": it is dihydrogen.

Identifying certain ions

Metal ion tests: 

A few drops of sodium hydroxide (NaOH) are added to a solution containing metal ions. A characteristic coloured precipitate forms.

  • $Cu^{2+}$ → blue precipitate
  • $Fe^{2+}$ → green precipitate
  • $Fe^{3+}$ → rust/orange precipitate
  • $Zn^{2+}$ → white precipitate
  • $Al^{3+}$ → white precipitate

  Example If sodium hydroxide is poured into a copper sulphate solution, a blue precipitate appears: copper(II) ions are present.

Chloride ion test ($Cl^-$): 

A few drops of silver nitrate ($AgNO_3$) are added to a solution that may contain chloride ions. A white precipitate appears which turns black in the light.

  Example If a solution of dissolved table salt is tested, adding silver nitrate produces a white precipitate that darkens: chloride ions are indeed present.

Sulphate test ($SO_4^{2-}$): 

A few drops of barium chloride ($BaCl_2$) are added to a solution that may contain sulphate ions. A white precipitate forms.

  Example: If barium chloride is poured into a copper sulphate solution, a white precipitate forms: sulphate ions are present.

 

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