Chemicals from seawater

Chemicals from seawater

Class practical

This experiment is a simple one to carry out and is designed to show that seawater contains a mixture of different salts. It can be used in conjunction with Earth Science topics linked to the conveyance of mineral salts into the sea via rivers.

This experiment could be carried out early in a science course while using scientific apparatus is still a novelty, because it provides practice at using a variety of equipment and also introduces the concept of a mixture and some simple chemical tests. It is also an opportunity to reinforce safety messages eg wearing eye protection throughout and activity and the need to keep standing up.
 

Lesson organisation

The experiment can be carried by groups of two or three and will take about one hour to complete.

Chemicals

Seawater, 200 cm³ (Note 1)

Access to hydrochloric acid, 1 M .

Apparatus

Eye protection

Beaker (250 cm³)

Beaker (100 cm³)

Conical flask (100 cm³)

Filter funnel

Filter paper

Bunsen burner

Heat resistant mat

Tripod

Gauze

Teat pipette

Health & Safety and Technical notes

Wear eye protection throughout. Take care with hot apparatus and solutions. 

Hydrochloric acid, HCl(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Limewater, Ca(OH)₂(aq), (treat as IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

Calcium sulfate, hydrated, CaSO₄.2H₂O(s) - see CLEAPSS Hazcard.

Sodium chloride, NaCl(s) - see CLEAPSS Hazcard. 

1 Although it is tempting to use genuine seawater if available, this experiment is usually more successful if the seawater is generated artificially. Genuine seawater will not always yield a full range of solids in sufficient quantities to be detected. Artificial seawater can be generated by adopting the following procedure:

• Bubble carbon dioxide through a mixture of 250 cm³ limewater plus 750 cm³ deionised water for about 20 minutes or until the cloudy precipitate disappears completely.
• Filter.
• Add as much solid hydrated calcium sulfate as will dissolve.
• Add about 15 g of sodium chloride.
• Stir until all the solid has dissolved, leave to settle and then decant the liquid if necessary.

Procedure

a Place 200 cm³ of seawater in a 250 cm³ beaker.

b Heat and boil the seawater.

c Stop heating when about 60-70 cm³ of liquid remains. Solid will be precipitated during this evaporation process.

d Allow to cool and for any solids to settle.

e Pour the clear liquid into the 100 cm³ beaker, leaving the solids behind.

f Add a few drops of hydrochloric acid to the solids left behind and observe what happens.

g Put the 100 cm³ beaker on the tripod and gauze and heat the liquid until another solid appears. This will occur when about 30-40 cm³ of liquid remains.

h Carefully filter the liquid into the conical flask.

i Wash out the 100 cm³ beaker and pour the filtrate into the beaker.

j Boil the liquid yet again until there is almost none left.

k Let it cool and note what is observed.

Teaching notes

Encourage the students to write down what they observe at each stage.

Teachers may wish to mark 250 cm³ beakers at the 60 cm³ level if there are no gradations already present.

The artificial seawater contains calcium hydrogencarbonate owing to the reaction of limewater with excess carbon dioxide:

Ca(OH)₂ (aq) + 2CO₂(g) → Ca(HCO₃)₂(aq)

When this solution is boiled it soon precipitates calcium carbonate:

Ca(HCO₃)₂(aq) → CaCO₃(s) + CO₂(g) + H₂O(l)

This is the identity of the predominant solid first left behind when the liquid is boiled. However, some calcium sulfate will also be present.

When hydrochloric acid is added to this solid, students should observe effervescence (fizzing), since the calcium carbonate is producing carbon dioxide gas:

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)

The solid which continuously crystallises out on further evaporation is sparingly soluble calcium sulfate, which is the predominant solid filtered off when 30 cm³ of sea water remains.

The more soluble sodium chloride precipitates out during the final stages of evaporation.

Student questions
Here are some questions for students.

a What evidence is there that seawater is a mixture of salts?

b What gas is likely to have been evolved when hydrochloric acid is added to the solids first collected?

c What does this tell you about the identity of these solids?

d Research the web to try to find information about the solubilities of sodium chloride and calcium sulfate – two common compounds present in seawater. Use this information to predict the possible identity of the very last solid left at the end of your experiment.