Precipitation
The process of precipitation usually involves the mixing of two clear solutions to produce an insoluble and often coloured solid.
· Set up a table similar to the one shown below.
Chemical

Silver
nitrate

Lead
nitrate

Copper
sulfate
Potassium iodide Sodium carbonate Sodium chloride Potassium chromate Barium
chloride
Silver
nitrate
             
Lead
nitrate
             
Copper
sulfate
             
Potassium iodide Yellow precipitate            
Sodium carbonate              
Sodium chloride              
Potassium chromate              
Barium
chloride
               
Procedure
· Mix the solutions as per table in a micro-test tube.
· Observe any solid that may be formed.
· If a precipitation reaction does occur name the precipitate and describe its appearance.

Naming the precipitate
- Simply swap the first names of the two solutions.
Eg. When lead nitrate and potassium iodide solutions are mixed.
Swaping the first names we come up with the names
Lead iodide
Potassium iodide.
Now keep in mind that if the name of a compound contains the words, potassium, nitrate, sodium or ammonium it can not be a precipitate as these compounds are always soluble and precipitates need to be insoluble.


What is the name of the precipitate formed when silver nitrate solution is mixed with sodium chloride solution?
If quantities of solutions is scarce than use a black tile and place drops of each solution in a grid as indicated by the table.
If quantities of solutions are scarce then use a black tile and place drops of each solution in a grid as indicated by the table.
A black tile makes it easy to see the colour change.
Exercise - A student mixed solutions of metal and non-metal ions and recorded her results in the the table shown below.
Carbonate ions
Chloride ions
Sulfide ions
Barium
Precipitate formed
Cadmium
Precipitate formed
Precipitate formed
Silver
Precipitate formed
Precipitate formed
Precipitate formed

The student has a solution of dissolved barium, cadmium and silver. She wishes to separate solid compounds of each metal from the solution by filtering the precipitates formed.

In what order must she add the solutions of carbonate, chloride and sulfide so that she filters out a compound of each metal in turn?

A chemical contractor is suspected of pouring dangerous chemicals into the river system and polluting the local water supply. Heavy metals appeared in the water and caused serious health problems in the local community. In trying to identify the metals present, a study was undertaken. Samples of water were collected and tested with the following chemicals.

Potassium iodide gave a yellow precipitate. What metals could be present? Explain

A sample of water is suspected of having lead or silver, but not both. Describe a series of reactions that can be used to identify the metal.


 

 

A sample of storm water, from an industrial estate, was tested with four solutions. The results for three of those solutions is shown on the right. Testing with lead nitrate solution did not produce a precipitate. Using the results of your experiment identify the likely contaminant found in the storm water.

Click on the picture to enlarge it.

 

An unknown, clear, odourless solution was left on the bench of the laboratory. It was tested with two solutions shown on the right. Is it safe to drink? Explain.

Silver iodide is a precipitate that can be formed between

The crystal structure of AgI is similar to that of ice, allowing it to induce freezing of water in clouds . Silver iodide is used in cloud seeding.

What property of silver iodide makes it useful in photography?

 

White precipitates are very common. A sample of water from a nearby river was tested with a chloride solution. A white precipitate is formed. The locals suspect that lead is present. Suggest further tests that can be carried out and what results can be expected to show the presence or absence of lead.

A Jewellery store was robbed of silver valued at one million dollars. The silver was dissolved by chemical means and sold. A suspect was arrested and his clothing tested. Describe two tests that can be conducted to see if silver metal is present on the suspect's clothing.

Another clever use of precipitation reactions was during WWII to camouflage submarines. Click to read.

Continue with naming precipitates.
An extension activity could be designed where students are asked to identify metal ions in given solutions. Solutions with mixtures of barium nitrate, copper nitrate, lead nitrate and or silver nitrate are prepared and made available to students. Students use the data collected in the above experiment to identify the metal ions present.
Name the solutions that can be mixed together to produce a pure sample of calcium carbonate?
How could you purify the sample of calcium carbonate formed in the question above?

The picture on the right shows a red precipitate formed when silver nitrate is mixed with potassium chromate.

What is the name of the precipitate in the filter paper?

Explain why it is not a pure sample.

How can we obtain a pure sample of this precipitate?

Using solutions of lead nitrate and ammonium sulphate describe a technique where by a pure, solide sample of ammonium nitrate can be obtained.
Continue with an application.

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