Acids Preparing soluble salts — Acids — Edexcel Combined Science 1SC0

Recall that acids in solution provide hydrogen ions, while alkalis in solution provide hydroxide ions.
Recall that neutral solutions have a pH of 7, acidic solutions have pH values below 7, and alkaline solutions have pH values above 7.
Recall the specific colour changes of indicators, including litmus, methyl orange, and phenolphthalein, when exposed to acids and alkalis.
Understand the inverse relationship between hydrogen ion concentration and pH in acidic solutions, and the direct relationship between hydroxide ion concentration and pH in alkaline solutions.

Choosing a method for a soluble salt

A soluble salt dissolves in water. When it is made by reacting an acid with a base, the immediate product is a salt solution, not dry crystals. Preparing the crystals therefore involves two jobs: making a solution without leftover reactants, then removing enough water for the salt to crystallise.

The method depends on whether the reactant added to the acid is soluble. An insoluble solid can be added in excess and the leftover solid filtered out. A soluble reactant cannot be removed this way: it passes through filter paper along with the salt solution. Instead, a titration finds the correct reacting volumes.

Making copper sulfate solution from an insoluble base

Copper(II) oxide is a black, insoluble solid. It acts as a base and reacts with dilute sulfuric acid to produce copper(II) sulfate and water. This is a neutralisation reaction.

Copper(II) oxide + sulfuric acid → copper(II) sulfate + water

\mathrm{CuO(s) + H_2SO_4(aq) \rightarrow CuSO_4(aq) + H_2O(l)}

Initially, added copper oxide reacts and disappears, producing a blue solution. Adding more until some black solid remains after stirring makes the copper oxide excess. This ensures that all the acid has reacted: leftover acid would contaminate the salt solution.

The excess copper oxide is insoluble, so filtration removes it. The black residue stays on the filter paper, while the blue filtrate passes through. The filtrate contains dissolved copper sulfate and water, with neither excess acid nor excess copper oxide.

Core practical: preparing hydrated copper sulfate crystals

The apparatus includes a measuring cylinder, conical flask, water bath, spatula, stirring rod, filter funnel and paper, beaker, evaporating basin and Petri dish or watch glass. For heating the evaporation water bath, use a Bunsen burner, tripod, gauze and heat mat. Tongs allow hot apparatus to be handled safely.

Measure about 20 cm³ of dilute sulfuric acid into a conical flask. Place the flask in a 50 °C water bath for 3–4 minutes. Warming increases the rate of reaction; the water bath provides gentle, controlled heating.
Add copper oxide in small portions with a spatula, stirring or swirling between additions. Continue until black powder remains after stirring.
Return the mixture to the water bath for a few minutes so that any remaining acid can react with the excess copper oxide.
Filter the mixture into a beaker, then transfer the blue filtrate to an evaporating basin.
Place the basin on top of a beaker half full of water. Support the beaker on gauze and a tripod above a heat mat, and heat it with a Bunsen burner on a blue flame. The hot water heats the basin indirectly.
Stop heating when about half the water has evaporated, then allow the basin to cool.
Transfer the cooled solution to a Petri dish or watch glass. Leave it for a few days so that further water evaporates and crystals form.

Mixture is poured into a filter-paper-lined funnel. Black excess copper(II) oxide remains as residue on the paper, while blue copper sulfate solution passes through into the beaker as filtrate. In the second panel, an evaporating basin rests on a beaker half full of water, supported by gauze and a tripod above a Bunsen burner. Heat passes indirectly through the water bath to concentrate the solution. Stop after about half the solution’s water has evaporated, then cool; do not heat to dryness. Apparatus is not to scale. — Filtration removes insoluble copper oxide; a water bath then gently concentrates the copper sulfate solution.

Evaporation makes the solution more concentrated. Cooling and further evaporation allow dissolved copper sulfate to form solid crystals. Do not keep heating until the basin is dry: strong heating can damage the hydrated product, and concentrated solutions can spit.

The final crystals are blue hydrated copper sulfate. Hydrated means that water molecules are part of the crystal structure; this is called water of crystallisation. A dry hydrated crystal has no liquid water coating its surface, but still contains its water of crystallisation. Leaving the remaining liquid to evaporate produces dry crystals without strong heating.

Record the appearance of the starting materials and the final solution, then the colour, shape and size of the crystals. The acid is colourless, copper oxide is black, and the copper sulfate solution and hydrated crystals are blue. Describe the crystal shapes and sizes actually obtained rather than assuming every preparation produces identical crystals.

Wear eye protection throughout. Avoid contact with the acid and copper compounds, keep the water bath stable, and handle hot apparatus carefully. Tie back long hair and keep flammable materials away from the Bunsen flame.

Why a soluble reactant needs titration

To prepare sodium chloride, hydrochloric acid can react with sodium hydroxide solution:

\mathrm{HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)}

Sodium hydroxide is an alkali: a soluble base. If it is added in excess, the extra sodium hydroxide remains dissolved. Filtration cannot separate it from the sodium chloride. Nor can the correct proportions be judged from the appearance of these colourless solutions.

A titration solves this problem by measuring the volume of acid needed to neutralise a fixed volume of alkali. A suitable indicator makes the end point visible. For hydrochloric acid added to sodium hydroxide, phenolphthalein changes from pink to colourless at the end point.

Carrying out the titration

A pipette measures a fixed volume accurately. A burette delivers a variable volume accurately, controlled by a tap.

Rinse the pipette with the sodium hydroxide solution, discarding the rinsing liquid. Use a pipette filler to transfer a fixed volume into a conical flask. Never pipette by mouth.
Add a few drops of phenolphthalein. The alkaline solution is pink.
Clamp the burette vertically and fill it with dilute hydrochloric acid, with the top below eye level. Remove the filling funnel and record the initial reading, reading the bottom of the meniscus at eye level.
Add acid to the flask while swirling to mix the solutions. Near the end point, temporary colour changes take longer to disappear. Add the acid drop by drop until the pink colour just disappears and does not return after swirling.
Record the final burette reading. The volume delivered is the titre:

\text{titre} = \text{final burette reading} - \text{initial burette reading}

A pipette fixes the alkali volume; the burette measures the acid needed to reach the indicator's end point.

Repeat the titration to check the volume needed. The correct reacting proportions depend on the solutions used; they are not necessarily equal volumes.

Turning the titration result into a pure, dry salt

The solution from the titration contains indicator, so it is not suitable for making a pure salt. Prepare a fresh mixture using the same fixed volume of alkali and the measured neutralising volume of acid, but no indicator.

These proportions use up both reactants, leaving only sodium chloride and water. Gently evaporate some water, then allow the concentrated solution to cool and crystallise. Let the remaining liquid evaporate to obtain dry salt. Sodium chloride crystals, unlike hydrated copper sulfate crystals, contain no water of crystallisation.

Choose the method

Insoluble reactant: add excess → filter → crystallise.
Soluble reactant: titrate → mix the correct volumes without indicator → crystallise.

Copper sulfate core practical

\mathrm{CuO(s) + H_2SO_4(aq) \rightarrow CuSO_4(aq) + H_2O(l)}

Link excess insoluble reactant to using up all the acid, then link filtration to removing the unreacted solid.

The copper sulfate is in the filtrate, not the residue: dissolved salts pass through filter paper.

For titration, use the measured reacting volumes, not necessarily equal volumes of acid and alkali.

Repeat the titration reaction without indicator when preparing the salt; otherwise the indicator contaminates the product.

Dry hydrated crystals have no liquid water on their surface but still contain water of crystallisation. Do not heat them strongly to remove this water.

A substance formed when the hydrogen in an acid is replaced by a metal or ammonium ion.

A reactant supplied in a greater amount than is needed to react completely with the other reactant, so some remains unreacted.

The liquid that passes through filter paper during filtration.

The insoluble solid left on filter paper during filtration.

The formation of solid crystals from a solution, for example as water evaporates or a concentrated solution cools.

Containing water molecules as part of a crystal's structure.

Water molecules incorporated into the structure of a hydrated crystal.

A method of finding the volume of one solution needed to react completely with a measured volume of another solution.

The point in a titration at which the indicator shows its lasting colour change.

The volume of solution delivered from a burette during a titration, calculated as final reading minus initial reading.

A substance formed when the hydrogen in an acid is replaced by a metal or ammonium ion.

A reactant supplied in a greater amount than is needed to react completely with the other reactant, so some remains unreacted.

The liquid that passes through filter paper during filtration.

The insoluble solid left on filter paper during filtration.

The formation of solid crystals from a solution, for example as water evaporates or a concentrated solution cools.

Containing water molecules as part of a crystal's structure.

Water molecules incorporated into the structure of a hydrated crystal.

A method of finding the volume of one solution needed to react completely with a measured volume of another solution.

The point in a titration at which the indicator shows its lasting colour change.

The volume of solution delivered from a burette during a titration, calculated as final reading minus initial reading.

Warm about 20 cm³ dilute sulfuric acid in a 50 °C water bath for 3–4 minutes.
Add copper oxide in small portions, stirring, until black solid remains. Warm again briefly.
Filter: excess copper oxide is the residue; copper sulfate solution is the blue filtrate.
Heat the filtrate in an evaporating basin over a water bath until about half the water has evaporated.
Cool, transfer to a watch glass or Petri dish, and leave for a few days to form dry, blue hydrated crystals.

Reasons: warming speeds the reaction; excess oxide uses up all acid; filtration removes excess solid; evaporation concentrates the solution.

Dry ≠ anhydrous: hydrated crystals retain water of crystallisation. Avoid strong heating or heating to dryness. Wear eye protection and take care with hot apparatus.

Titration and salt preparation

Pipette + filler: fixed volume of alkali into a conical flask.
Burette: acid added while swirling; add dropwise near the end point.
Suitable indicator: for hydrochloric acid and sodium hydroxide, phenolphthalein changes pink → colourless.

\text{titre} = \text{final reading} - \text{initial reading}

Repeat to check the volume. Mix fresh solutions in the measured proportions without indicator, then concentrate, crystallise and dry.

\mathrm{HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)}

Excess soluble reactant cannot be filtered out. Correct proportions leave only salt and water.

Preparing soluble salts

What you need to know