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Why is it impossible to produce an insoluble salt between a solid and a liquid?

For example in the reaction between sulfuric acid and calcium carbonate the calcium sulfate produced is almost insoluble in water. As soon as the reaction starts a layer of calcium sulfate form around the carbonate preventing any further reaction. For this reason it seems as if the calcium carbonate doesn't react with the acid. If the salt was soluble a calcium sulfate solution would form.


Name the solubility patterns:

• All salts containing ammonium, potassium and sodium are soluble.
• most carbonates and metal hydroxides are insoluble EXCEPT ammonium, potassium and sodium carbonate
• All nitrates soluble
• most chlorides soluble EXCEPT lead chloride and silver chloride
• most sulfates soluble EXCEPT lead sulfate and barium sulfate


What three mixtures can you use for making soluble salts?

Excluding potassium, sodium and ammonium salts you can react a solid with an acid.
• acid + metal
• acid + metal oxide or hydroxide
• acid + carbonate
whatever mixture you use the method is essentially the same


Describe the excess base method

1) measure out the volume of acid with a measuring cylinder into a beaker. Weigh out the insoluble base and add to the acid. Warm gently, but do not boil.
1) continue heating and adding the base until it is in excess. This means that all the acid has been neutralised
2) when the solution is cool, filter it using a filter funnel and paper to remove the excess base
3) transfer to an evaporating basin and leave the solution to crystallise
4) any uncrystallised solution can be poured off the crystals and the crystals can be blotted dry with tissue paper


Why not just evaporate the solution to dryness after the excess base method?

You will trap water in the crystals and they will be impure


What does anhydrous mean?

'without water'


What is water of crystallisation?

when many salts form their chrystals water from the solution becomes chemically bound up with the salt. This is called water of crystallisation. A salt that contains water of crystallisation is said to be hydrated.


How do you know whether the acid should be hot or not?

Carbonates and magnesium react with dilute acids in the cold. Most other things must be heated.


Why is the method of making soluble salts with sodium, potassium and ammonium different?

The solids are soluble in water so they would not only react with the acid but any excess solid would dissolve in the water present so it would not be clear how much excess there is.


How can you make soluble salts with sodium potassium and ammonium?

Using titration. All of these salts are alkaline as they are hydroxide/carbonate so you can see how much is needed to neutralise the acid using an indicator.


What is the point at which the indicator changes colour in the titration called?

The end point of the titration


Describe the process of making soluble salts with sodium potassium and ammonium (titration method)

1) place 25cm3 of an acid in a conical flask using a pipette and add a few drops of an indicator (not universal as we want a gradual change)
2) run an alkali from the burette until the indicator turns pH7 neutral. This means that all the acid has been neutralised
3) note the volume of alkali and repeat steps 1 and 2the same volume of acid and alkali are used in a clean flask with no indicator (as this is contaminating)
4) the solution is transferred to an evaporating dish and left. The water will evaporate, forming crystals.
5) separate the crystals from any remaining solution and allow them to dry.


What is the name of the reaction that makes insoluble salts?

Precipitation reaction


What is a precipitate?

A solid that is formed by a chemical reaction involving liquids or gases.


What is happening when a precipitate forms?
(long answer for understanding, I think not necessary)

For example if silver nitrate is added to sodium chloride, silver chloride is formed. The positive silver ions and negative nitrate ions were attracted to each other but the attractions were not strong enough to make them stick together. Similarly with the sodium chloride solution.
When you mix the two solutions various ions meet each other. When the silver ions come in contact with the chloride ions they are very strongly attracted to each other so the ions clump together and form a solid as it does not dissolve in water. The nitrate ions and sodium ions remain in the solution because they are not sufficiently attracted to each other. They are the spectator ions.


What is the full equation for the precipitation reaction of silver nitrate and sodium chloride?
What is the ionic equation?

AgNO3(aq) + NaCl(aq) ---> AgCl(s) + NaNO3(aq)

Ag+(aq) + Cl-(aq) ---> AgCl(s)


Describe the precipitation method to get an insoluble salt

1) mix together two solutions e.g. AgNO3 and NaCl, forming the precipitate AgCl(s).
2) the salt (precipitate) is obtained by filtering the mixture to get rid of the solution
3) then wash the salt with pure water to remove traces of the other solutions
4) finally the salt is left to dry


In summary how do you make an insoluble salt?

Using a precipitation method: mix two solutions, one containing the correct positive ion and one containing the correct negative ion.


If you are doing the titration experiment, what does it mean if your reaction mixture is too acidic?

If the reaction mixture is acidic then you have added too much acid after the reaction is already complete, therefore the acid becomes an impurity.


How do you decide what method to use?

Is it soluble? No - precipitation method
Yes - Is it a sodium, potassium or ammonium salt? Yes - titration method No - excess base method