C2 - Analysing Ionic Compounds

Precipitate = Insoluble solid formed in a precipitate reaction

Precipitate reaction = When soluble reactants react to form a insoluble product

Since two products are formed, you can work out which one is the insoluble salt by using the solubility rules.

For example:

Silver Nitrate + Potassium Chloride → Silver Chloride + Potassium Nitrate

AgNO₃ (aq) + KCl (aq) → AgCl (s) + KNO₃ (aq)

The precipitate is silver chlroide - it is a solid.

Barium sulfate is used with patients who are going to have their stomach or instestines X-rayed because barium sulfate is opaque to X-rays, meaning that it shows up well on the X-ray picture.

A "barium meal" is given to the patients who are then X-rayed.

The barium sulfate will show the shape of the instestine on the X-ray so that the doctor can tell whether or not there is any problems like growth or lumps.

Even though barium salts are toxic, barium sulfate is insoluble - it will not get into the patient's blood.

Sodium Chloride (NaCl)

• Has a high melting point (800°c)
• Cannot conduct electricity when solid but when molten or dissolved in water, will conduct electricity

Magnesium Oxide (MgO)

• Has a higher melting point than sodium chloride (around 2,800°c) because the Mg and O ions form a stronger ionic bond than Na and Cl as the Mg and O have a higher number of charges.
• Since it stays solid at such a high temperature, it remains non-conductive.
• Therefore it is used in high-temperature electrical insulation

Soluble salts (dissolve in water)

• All nitrates
• Most sulfates
• Most chlorides, bromides and iodides
• Sodium carbonate, Potassium carbonate and Ammonium carbonate
• Sodium hydroxide, Potassium hydroxide and Ammonium hydroxide

Insoluble salts (do not dissolve in water)

• Lead sulfate, Barium sulfate, and Calcium sulfate
• Silver chloride, Silver bromide, silver iodide
• Lead chloride, Lead bromide and Lead iodide
• Most carbonates (except the ones mentioned as soluble)
• Most hydroxides (except the ones mentioned as soluble)

Some metal cations can be identified using a flame test.

A damp splint is placed in the solid and then held in the Bunsen burner flame.

The colour of the flame identifies the ions.

• Calcium - Brick red
• Lithium - Red
• Sodium - Yellow
• Copper - Green/Blue
• Barium - Pale Green
• Potassium - Lilac

When heated, all atoms give off light but sometimes it is not visible to the human eye.

A prism can be used to split this light into a spectrum - each element has its own distinctive line spectrum

Spectroscopys allow chemists to detect a very small amount of an element.

Rubidum and caesium were discovered when scientists saw new colours in parts of the spectrum they had not seen before.

Helium was discovered by studying the line of spectra emitted by the sun.

Sulfate ions

1. Add a few drops of dilute hydrochloric acid

2. Invert the mixture

3. Add a few drops of barium chloride solution

If the substance contains sulfate ions, a white precipitation of barium sulfate forms. This is because barium sulfate is insoluble in water.

Pure samples of insoluble salts can be prepared using precipitation reactions.

1. Mix solutions of two substances that will form the insoluble salt.

2. Filter the mixture - the insoluble salt will be trapped in the filter paper.

3. Wash the salt with pure water.

4. Leave the salt to dry on the filter paper - it could even be dried in an oven.

Carbonate ions

1. Add dilute acid to the unknown substance.

2. Using limewater, test the gas given off.

If it fizzes or bubbles, carbonate ions are present because a gas has been given off.

Carbonate ions are also present when the gas turns clear limewater cloudy. This indicates that the gas is carbon dioxide.

Chloride ions

1. Add a few drops of dilute nitric acid to the solution.

2. Invert the mixture.

If a white precipitate of silver chloride forms, it is an indication of chloride ions being present.