Solubility of salts
Definition
Soluble
Able to dissolve in a given solvent, which is water unless another solvent is named.
Definition
Insoluble
Unable to dissolve in a given solvent, which is water unless another solvent is named.
- A salt is soluble if it dissolves in water and insoluble if it does not.
- Whether a salt is soluble decides which method is used to make it, so the general rules are worth learning by heart.
- Sodium, potassium and ammonium salts: all of them are soluble.
- Nitrates: all of them are soluble.
- Chlorides: soluble, except lead(II) chloride and silver chloride.
- Sulfates: soluble, except barium sulfate, calcium sulfate and lead(II) sulfate.
- Carbonates: insoluble, except sodium, potassium and ammonium carbonate.
- Hydroxides: insoluble, except sodium, potassium and ammonium hydroxide, with calcium hydroxide only partly soluble.
- Most metal oxides are also insoluble in water, which makes them useful as bases for preparing salts.
Example
Applying the solubility rules
- Copper(II) sulfate is a sulfate and copper is not one of the exceptions, so it is soluble.
- Silver chloride is one of the two insoluble chlorides, so it is insoluble.
- Zinc carbonate is a carbonate and zinc is not sodium, potassium or ammonium, so it is insoluble.
- Barium nitrate is a nitrate, so it is soluble, even though barium sulfate is insoluble.
Choosing a method
- A soluble salt is made by reacting an acid with a substance that contains the metal you need in the salt.
- The acid sets the second part of the name, so hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates.
- The method then depends on whether the other reactant dissolves in water.
- Excess insoluble solid
- This method is used when the other reactant is a metal, an insoluble base or an insoluble carbonate.
- Any unreacted solid can be removed by filtration, because it does not dissolve.
- Zinc sulfate can be made from zinc, copper(II) sulfate from copper(II) oxide, and magnesium chloride from magnesium carbonate.
- Titration
- This method is used when the other reactant is an alkali, so it is the method for sodium, potassium and ammonium salts.
- An alkali dissolves in water, so any excess could not be filtered off and would be left mixed with the salt.
- A metal is only suitable if it reacts with dilute acid at a safe rate, so sodium and potassium are too reactive and copper does not react at all.
- Copper salts are therefore made from copper(II) oxide or copper(II) carbonate rather than from copper metal.
Reacting an acid with an excess of solid
Definition
Filtrate
The liquid that passes through the filter paper during filtration.
Definition
Residue
The substance left behind after a process such as filtration, evaporation or distillation.
- The insoluble solid is added to the acid in excess, which means more is added than can react.
- Using an excess makes sure that all of the acid is used up, so the solution left contains only the salt dissolved in water.
- With a carbonate, the acid is used up when the fizzing stops and some solid remains.
- With a metal, the acid is used up when hydrogen bubbles stop forming and some metal remains.
- With a base, the acid is used up when no more solid dissolves, even after warming and stirring.
- Warming the acid gently speeds up the reaction with an insoluble base such as copper(II) oxide.
- The mixture is then filtered, so the excess solid stays on the filter paper as the residue and the salt solution passes through as the filtrate.
Crystallising the salt
Definition
Saturated solution
A solution that holds the maximum amount of dissolved solute possible at a given temperature.
Definition
Crystallisation
The formation of solid crystals from a saturated solution as it cools.
- The filtrate is heated in an evaporating basin to boil off some of the water and make the solution more concentrated.
- Heating stops at the crystallisation point, when the solution is saturated.
- You can find this point by dipping a glass rod into the hot solution and lifting it out, because small crystals form on the rod as the liquid on it cools.
- The saturated solution is left to cool slowly, and crystals form because less salt can stay dissolved in cold water than in hot water.
- Slow cooling gives larger and better-shaped crystals than fast cooling.
- The crystals are separated from the remaining liquid by filtration.
- They are washed with a small amount of cold distilled water to rinse away the remaining solution, which carries any soluble impurities.
- Cold water is used because less of the salt dissolves in it, so fewer crystals are lost while washing.
- The crystals are then dried by pressing them between filter papers or leaving them in a warm place.
- The solution is not evaporated to dryness, because heating too strongly can break down the crystals and leaves any soluble impurities in the solid.
Practical
Preparing copper(II) sulfate crystals
- Aim: to prepare pure, dry crystals of hydrated copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.
- Apparatus: 100 cm3100\ \text{cm}^3100 cm3 beaker, 25 cm325\ \text{cm}^325 cm3 measuring cylinder, dilute sulfuric acid, copper(II) oxide powder, spatula, glass rod, Bunsen burner, tripod, gauze, heat-resistant mat, filter funnel, filter paper, conical flask, evaporating basin, beaker of water to act as a water bath, extra filter papers and eye protection.
- Equation: CuO(s)+H2SO4(aq)→CuSO4(aq)+H2O(l)\text{CuO(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{CuSO}_4\text{(aq)} + \text{H}_2\text{O(l)}CuO(s)+H2SO4(aq)→CuSO4(aq)+H2O(l)
- Method:
- Measure 25 cm325\ \text{cm}^325 cm3 of dilute sulfuric acid into the beaker and warm it gently on a tripod and gauze, without letting it boil.
- Add copper(II) oxide one spatula at a time, stirring after each addition, and watch the black powder disappear as the colourless acid turns blue.
- Keep adding until some black solid stays at the bottom even after stirring, which shows that the copper(II) oxide is in excess and the acid has all reacted.
- Filter the warm mixture into a conical flask, so the excess black solid stays on the filter paper and a clear blue filtrate collects.
- Pour the filtrate into an evaporating basin and heat it over a water bath until about half of the water has evaporated.
- Dip a glass rod into the solution and lift it out, and stop heating as soon as small crystals appear on the rod as it cools.
- Leave the basin in a cool place for a day so that crystals grow slowly.
- Pour off the remaining liquid, rinse the crystals with a little cold distilled water and pat them dry between filter papers.
- Results: bright blue, diamond-shaped crystals of hydrated copper(II) sulfate form in the basin.
- Adapting the method: the same steps make zinc sulfate from excess zinc, or magnesium chloride from excess magnesium carbonate and dilute hydrochloric acid, and here the end of the reaction is shown when the fizzing stops.
- Safety: wear eye protection, take care with hot acid, avoid skin contact with copper(II) sulfate because it is harmful, and never heat the basin to dryness because the hot solid can spit.
Making a salt by titration
Definition
Titration
A method in which one solution is added gradually from a burette to a measured volume of another solution until the reaction between them is just complete.
Definition
End-point
The point in a titration at which the indicator changes colour, showing that the reaction is just complete.
- A titration adds just the right volume of acid to an alkali, so neither reactant is left over in the salt.
- A volumetric pipette measures an exact volume of the alkali, such as 25.0 cm325.0\ \text{cm}^325.0 cm3, into a conical flask.
- A few drops of a suitable indicator, such as methyl orange or thymolphthalein, are added to the flask.
- The acid is added from a burette while the flask is swirled, until the indicator just changes colour.
- This colour change is the end-point, and it shows that the acid and alkali have exactly reacted.
- The volume of acid used is found from the burette readings before and after.
- The indicator is now an impurity in the salt solution, so the titration is repeated with the same volumes of acid and alkali but without the indicator.
- The pure salt solution is then crystallised in the same way as before.
- Universal indicator is not used, because its colour changes gradually and does not give one sharp end-point.
Practical
Preparing sodium chloride by titration
- Aim: to prepare pure crystals of sodium chloride from sodium hydroxide solution and dilute hydrochloric acid.
- Apparatus: burette, clamp stand, small funnel, 25.0 cm325.0\ \text{cm}^325.0 cm3 volumetric pipette with a pipette filler, conical flask, white tile, dilute hydrochloric acid, dilute sodium hydroxide solution, methyl orange indicator, evaporating basin, Bunsen burner, tripod, gauze, filter papers and eye protection.
- Equation: NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l)\text{NaOH(aq)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l)
- Method:
- Rinse the burette with a little of the acid, then fill it using the funnel, remove the funnel and run a few drops out so the tip is full with no air bubble.
- Read the burette at eye level from the bottom of the meniscus and record the starting reading.
- Rinse the pipette with sodium hydroxide solution, then use the pipette filler to transfer exactly 25.0 cm325.0\ \text{cm}^325.0 cm3 into the conical flask on the white tile.
- Add three drops of methyl orange, which turns the solution yellow.
- Run in acid from the burette while swirling the flask, adding it drop by drop near the end, until the solution just changes from yellow to orange.
- Record the final reading, work out the volume of acid added, and repeat until two volumes agree closely.
- Mix a fresh 25.0 cm325.0\ \text{cm}^325.0 cm3 of sodium hydroxide solution with exactly that volume of acid, this time without any indicator.
- Heat the solution in an evaporating basin to the crystallisation point, then leave it to cool and crystallise.
- Filter off the crystals, rinse them with a little cold distilled water and dry them between filter papers.
- Results: small, colourless, cube-shaped crystals of sodium chloride form.
- Safety: wear eye protection because sodium hydroxide damages the eyes, fill the burette below eye level, and always use a pipette filler instead of sucking by mouth.
Hydrated and anhydrous salts
Definition
Hydrated substance
A substance that has water chemically combined within it.
Definition
Anhydrous substance
A substance that contains no water at all.
- Many salts crystallise with water molecules chemically combined into their crystals.
- A salt containing this chemically combined water is hydrated, while a salt with no water at all is anhydrous.
- Hydrated copper(II) sulfate forms blue crystals, while anhydrous copper(II) sulfate is a white powder.
- Hydrated cobalt(II) chloride is pink, while anhydrous cobalt(II) chloride is blue.
- Hydrated crystals look and feel dry, because the water is locked into the crystal structure rather than wetting the surface.
- Strong heating drives this water out of the crystals, which is another reason for crystallising a salt instead of evaporating its solution to dryness.
Common Mistake
- Do not make sodium, potassium or ammonium salts by the excess method, because the alkali dissolves and the excess cannot be filtered off.
- Do not evaporate a salt solution to dryness when you want hydrated crystals.
- Do not wash crystals with a large volume of warm water, because the salt dissolves and the crystals are lost.
- Do not use universal indicator in a titration, because it has no sharp end-point.
Exam technique
Describing a salt preparation
- Name both reactants and state which one is added in excess.
- Say how you know the reaction has finished, such as "no more fizzing" or "solid remains".
- Give the separation steps in order: filter, heat to the crystallisation point, cool, then filter, wash and dry the crystals.
- For a titration, say that the experiment is repeated without the indicator before crystallising.
Self review
- Which two chlorides are insoluble in water?
- Why is the insoluble base added in excess when making copper(II) sulfate?
- How do you know when a salt solution has reached the crystallisation point?
- Why are sodium salts made by titration rather than by the excess method?
- What is the difference between a hydrated salt and an anhydrous salt?
