Electrolytic dissociation of salt

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Link to the lesson

Before you start you should know

that salts are ionic compounds and their crystal lattice structure is composed of metal cations and anions of acidic residues;
that water‑soluble hydroxides undergo electrolytic dissociation (dissociate) into metal cations and hydroxide anions;
that the acids dissociate into hydrogen cations and anions of the acid residue.

You will learn

to note down the equations of the electrolytic dissociation process of salts.

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Are all salts soluble in water?

Task 1

Before you watch the movie „Testing solubility of salt in water”, formulate a research question and a hypothesis. Write down the observations and conclusions. During the screening pay attention to what happens to the salts while trying to dissolve them.

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Testing solubility of salt in water
Source: GroMar Sp. z o.o., licencja: CC BY-SA 3.0.

Are all salts soluble in water?

Experiment 1

Research problem

Do all salts dissolve in water to the same extent?

Hypothesis

Select one of the hypotheses and then verify it.

All salts dissolve in water.
There are salts well‑soluble in water and salts that are sparingly soluble.
The salts are insoluble in water.

You will need

beakers,
glass rods,
spoons,
water,
sodium chloride,
magnesium sulfate,
calcium carbonate,
calcium phosphate,
sodium carbonate.
magnesium carbonate.

Instruction

Pour identical water volume (50 cmIndeks górny 3) into seven similar beakers.
Into each of them pour 3 - 5 g of substance: into a beaker 1. – sodium chloride, into 2. - calcium phosphate, into a beaker 3. - magnesium carbonate, into 4. - calcium carbonate, into 5. - sodium carbonate. into 6. – magnesium sulphate.
Mix the contents of all beakers thoroughly using a glass rod.
Observe whether all solid substances are soluble in water.

Summary

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Observations

Conclusions

The salts do not dissolve in water to the same extent. Among them, there are salts very well soluble in water, such as, for example, potassium nitrate, and those that do not dissolve in it barely at all - e.g. barium sulphate. The same metal can form salts that are both well and sparingly soluble in water, e.g. calcium chloride and calcium carbonate. Similarly, salts containing the same anion of an acid residue may have different solubilities, e.g. calcium carbonate and sodium carbonate.

Where can I get information which salts are soluble in water?

Information on whether the salt will dissolve in water can be found in the solubility table. It will allow us to assess whether a given salt is well, sparingly or virtually insoluble in water. To determine this property for the selected salt, read the record at the intersection of the cation column with the row of anion (ions forming the salt of interest).

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Source: GroMar Sp. z o.o., licencja: CC BY-SA 3.0.

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Source: licencja: CC BY-SA 3.0.

All known salts of sodium, potassium, ammonium and all known nitrates are very well soluble in water.

Do aqueous solutions of salt conduct electricity?

Experiment 2

Research problem

Do aqueous solutions of salt conduct electricity?

Hypothesis

Select one of the hypotheses and then verify it.

Aqueous solutions of salt conduct electricity. Aqueous solutions of salt do not conduct electricity.

You will need

beakers,
glass rods,
spoons,
water,
device with a bulb to test the electrical conductivity of solutions,
calcium chloride,
sodium chloride (rock salt),
magnesium sulphate,
sodium carbonate,
potassium nitrate.

Instruction

Prepare five solutions of water soluble salts in the beakers, for example: calcium chloride, sodium chloride, magnesium sulphate, sodium carbonate, potassium nitrate.
Prepare a beaker with distilled water.
Test whether the prepared solutions conduct electricity by means of an electrical conductivity testing device.

Look at the interactive illustration. What conclusions can you draw after observing the experiment?

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Source: licencja: CC BY-SA 3.0.

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Observations

Conclusions

What happens to salts when dissolving in water?

Water‑soluble salts are electrolytes, their aqueous solutions conduct electricity. Under the influence of water, these disintegrate into the ions forming them, i.e. metal cations and anions of the acid residue. This process, as we remember, is called electrolytic dissociation.

During the dissolution of sodium chloride, polar water molecules release the ions from the crystal of the salt: sodium cations and chloride anions that can carry electric charge (electrons); due to this, the sodium chloride aqueous solution conducts electricity.

Task 2

Look at the table describing the electrolytic dissociation process of the selected salts. Perform the exercises – the table can help you solve some of these.

Name of salt	Molecular formula of salt	Equations of electrolytic dissociation of salts
sodium chloride	$NaCl$	$NaCl \overset{H_{2} O}{\to} {Na}^{+} + {Cl}^{–}$
potassium nitrate	$K {NO}_{3}$	$K {NO}_{3} \overset{H_{2} O}{\to} K^{+} + {NO}_{3}^{–}$
cupric sulphate (copper(II) sulphate)	$Cu {SO}_{4}$	$Cu {SO}_{4} \overset{H_{2} O}{\to} {Cu}^{2+} + {SO}_{4}^{2–}$
potassium phosphate	$K_{3} {PO}_{4}$	$K_{3} {PO}_{4} \overset{H_{2} O}{\to} {3K}^{+} + {PO}_{4}^{3–}$
lead(II) nitrate	$Pb {(NO}_{3})_{2}$	$Pb {(NO}_{3})_{2} \overset{H_{2} O}{\to} {Pb}^{2+} + {2NO}_{3}^{–}$
ferric sulphate (iron(III) sulphate)	${Fe}_{2} {(SO}_{4})_{3}$	${Fe}_{2} {(SO}_{4})_{3} \overset{H_{2} O}{\to} {2Fe}^{3+} + {3SO}_{4}^{2–}$

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Exercise 1

Based on the solubility table of hydroxides and salts, answer which salt is sparingly soluble in water. Select the correct answer.

iron(III) phosphate
potassium chloride
copper(II) sulphate
zinc nitrate

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Exercise 2

How many ions are formed during the dissociation of potassium phosphate? Select the correct answer.

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Exercise 3

If following ions: Na⁺, Cl^-, SO₃^2-, CO₃^2-, NO₃^- are present in water, then which salts could be dissolved? Select the correct salt set.

sodium chloride, sodium nitrate, sodium sulphite, sodium carbonate
sodium chloride, calcium carbonate, sodium sulphite, sodium carbonate
sodium chloride, calcium sulphite, sodium nitrate, sodium carbonate
sodium chloride, calcium sulphite, calcium carbonate, sodium carbonate

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Exercise 4

Organise the salt solutions according to the increasing number of the resulting ions formed during the dissociation of the smallest set of repeating ions in the crystals of these salts.

potassium phosphate solution
sodium sulphide solution
barium sulphide solution
iron(III) sulphate solution

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Exercise 5

Complete the text.

valence, metal, metal, acidic residues, non-metal, cation, metal, ide, acid residue, acid residue, ionic, lower

Salts are substances of .............................. structure, composed of .............................. cations and anions of ............................... The salt formulas are determined based on .............................. of .............................. and ............................... The valence of the metal is equal to the charge of the .............................. formed by it, and the valence of .............................. is equal to the charge of the anion formed by it. These numbers are inscribed on the cross as .............................. indexes. If the valences of ions are equal, then the corresponding indexes are equal to 1, and the number 1 in the formula of total salts is omitted. The salts of hydracids have an ending - .............................., and the salts of oxoacids have the ending – ate/-ite. When creating a systematic name of salt, the valence of .............................. and .............................. belonging to the acid residue is sometimes given, for example copper(II) phosphate.

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Exercise 6

Add the salts in the appropriate group:

lead(II) nitrate, silver chloride, aluminium chloride, tin(II) sulphide, iron(III) sulphate, calcium nitrate, sodium sulphide, potassium sulphite, aluminium carbonate, magnesium phosphate, iron(III) chloride, calcium carbonate, potassium phosphate, barium sulphate, barium sulphide

Salts of hydracids:
Salts of oxoacids:

Summary

There are salts that dissolve in water and salts that are sparingly soluble or practically insoluble in water.
Aqueous solutions of salt conduct electricity - salts are electrolytes.
Salts in water are subject to electrolytic dissociation, i.e. these disintegrate (dissociate) into metal cations and anions of the acid residue.

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Source: GroMar Sp. z o.o., licencja: CC BY-SA 3.0.

Homework

Task 3.1

Based on the information in the solubility table, determine which cations form the least water‑soluble salt. Similarly, evaluate the anions of acidic residues.

Keywords

electrolytic dissociation, salt, dissolving

Glossary

electrolytic dissociation of salts

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nagranie dźwiękowe słówka

dysocjacja elektrolityczna soli – sole pod wpływem wody rozpadają się na jony, z których są zbudowane, czyli na kationy metali i aniony reszty kwasowej

Lesson plan Polish

Are all salts soluble in water?

Where can I get information which salts are soluble in water?

Do aqueous solutions of salt conduct electricity?

What happens to salts when dissolving in water?

Summary

Keywords

Glossary