After completing this topic you should be able to : After completing this topic you should be able to :

• Describe effect of dilution on the pH of an acid or alkali is explained in terms of the decreasing concentration of hydrogen and hydroxide ions.

• Explain the low conduction in terms of dissociation of water molecules to form hydrogen and hydroxide ions.

• State in water and neutral solutions, the concentration of hydrogen ions is equal to the concentration of hydroxide ions.

• State an acidic solution contains more hydrogen ions than hydroxide ions and an alkaline solution contains more hydroxide ions than hydrogen ions.

DILUTING A SOLUTIONDILUTING A SOLUTION

During dilution of a solution the number During dilution of a solution the number of particles (ions or molecules) of solute of particles (ions or molecules) of solute in every cmin every cm33 decreases. decreases.

When a solution is mixed with water the When a solution is mixed with water the concentration of the solution decreases.concentration of the solution decreases.

How does diluting an acid or alkali affect the pH of the solution?How does diluting an acid or alkali affect the pH of the solution?

After each dilution the pH was measured After each dilution the pH was measured using a pH meter. using a pH meter.

pH meterpH meter

Concentration Concentration of of HCl(aq)HCl(aq)

(mol (mol ll-1-1))pHpH

0.10.1

0.010.01

0.0010.001

0.00010.0001

0.000010.00001

0.0000010.000001

0.00000010.0000001

11

22

33

44

55

66

77

Diluting an acid decreases the acidity and raises the pH towards 7.Diluting an acid decreases the acidity and raises the pH towards 7.

DILUTING AN ACIDDILUTING AN ACIDThe pH of 0.1 mol The pH of 0.1 mol ll-1-1 hydrochloric acid was measured using a pH meter. The acid hydrochloric acid was measured using a pH meter. The acid was then diluted by a factor of 10 by measuring 10 cmwas then diluted by a factor of 10 by measuring 10 cm33 of acid and adding 90 cm of acid and adding 90 cm33 of distilled water. The dilution was repeated several times.of distilled water. The dilution was repeated several times.

The table shows the results The table shows the results of the experiment.of the experiment.

CONCLUSIONCONCLUSION

DILUTING AN ALKALIDILUTING AN ALKALI

The table below shows the results when the pervious experiment was repeated The table below shows the results when the pervious experiment was repeated using 0.1 mol using 0.1 mol ll-1-1 sodium hydroxide. sodium hydroxide.

Concentration Concentration of of NaOH(aq)NaOH(aq)

(mol (mol ll-1-1))pHpH

0.10.1

0.010.01

0.0010.001

0.00010.0001

0.000010.00001

0.0000010.000001

0.00000010.0000001

1313

1212

1111

1010

99

88

77

Diluting an alkali decreases the alkalinity and Diluting an alkali decreases the alkalinity and results in the pH falling towards 7.results in the pH falling towards 7.

CONCLUSIONCONCLUSION

77 141411

When an acid and alkali are diluted enough the pH eventually arrives at pH 7.When an acid and alkali are diluted enough the pH eventually arrives at pH 7.

HH++ OHOH--HH++ OHOH--

DILUTION AND WATERDILUTION AND WATER

Pure water is a very poor conductor of electricity. This shows water contains Pure water is a very poor conductor of electricity. This shows water contains ions.ions.

The very low conductivity shows water contains a very small number of ions.The very low conductivity shows water contains a very small number of ions.

This shows that pure water contains a very small number of This shows that pure water contains a very small number of HH++ and and OHOH-- ions. ions.

WATERWATER

Water forms ions from its molecules in the following reaction.Water forms ions from its molecules in the following reaction.

HH22O(O(ll)) HH++

(aq)(aq)OHOH--(aq)(aq)++

The low conductivity of water shows very few molecules break up to form ions. The low conductivity of water shows very few molecules break up to form ions. Pure water is neutral because there are equal numbers of Pure water is neutral because there are equal numbers of HH++ andand OHOH-- ions.ions.

The double arrow shows the reaction is The double arrow shows the reaction is REVERSIBLEREVERSIBLE..

This reaction is called the This reaction is called the DISSOCIATION OF WATERDISSOCIATION OF WATER. . This means all solutions This means all solutions contain small numbers of contain small numbers of HH++ andand OHOH-- ions.ions.

Acids contain Acids contain MORE MORE HH++ ions than is present in pure water. ions than is present in pure water.

Alkalis contain Alkalis contain MORE MORE OHOH-- ions than is present in pure water. ions than is present in pure water.

Diluting an acid or alkali decreases the concentration of Diluting an acid or alkali decreases the concentration of HH++ oror OHOH-- to bring them to bring them down to the same concentration as present in pure water.down to the same concentration as present in pure water.