8.1 the characteristic properties of acids and bases

Chemistry IGCSE
chapter 8 Acids ,bases and salts

1. Chapter 8 Acids,Bases and Salts
Monday, April 6, 2020
By
Sir.Muhammad
Abdul Mageid
ChemistryTeacher

2. Contents
8.2 Types of Oxides
8.1 The Characteristic Properties of Acids and Bases
8.4 Identification of Ions and Gases
8.3 Preparation of Salts
Monday, April 6, 2020
Sir. Muhammad Abdul Mageid

3. Monday, April 6, 2020
8.1 The Characteristic Properties of Acids and Bases
Sir. Muhammad Abdul Mageid
Properties of Acids
•Acids have pH values of below 7, have a sour taste and are
corrosive.
•In acidic conditions, blue litmus paper turnsredand
methyl orangeindicatorturnsred.
•Acids are substances that can neutralise a base, forming
a salt and water.
•When acids react, they will lose electrons to form
positively chargedhydrogen ions(H
+
).
•The presence of H
+
ions is what makes a solution acidic.
Example:Hydrochloric Acid
HCl(aq) → H
+
(aq) + Cl
–
(aq)

4. Monday, April 6, 2020
Sir. Muhammad Abdul Mageid
8.1 The Characteristic Properties of Acids and Bases
Typical reactions of acids
1-Acids and metals
•Only metalsabove hydrogenin the reactivity series will react with
dilute acids.
•When acids react with metals they form a salt and hydrogen gas:
Acid + Metal → Salt + Hydrogen

5. Monday, April 6, 2020
Sir. Muhammad Abdul Mageid
8.1 The Characteristic Properties of Acids and Bases
2-Acids with Bases (Alkalis)
•Metaloxidesand metalhydroxidescan act asbases.
•When they react with acid, aneutralisationreaction occurs.
•Acids and bases will react together in a neutralisation reaction and produce
asaltandwater:
Acid + Base → Salt + Water

6. Monday, April 6, 2020Monday, April 6, 2020
Sir. Muhammad Abdul Mageid
8.1 The Characteristic Properties of Acids and Bases
3-Acids with Metal Carbonates
•Acids will react with metal carbonates to form the corresponding metal salt,
carbon dioxide and water:
Acid + Metal Carbonate → Salt + Carbon Dioxide + Water

7. Monday, April 6, 2020Monday, April 6, 2020
Sir. Muhammad Abdul Mageid
8.1 The Characteristic Properties of Acids and Bases
Properties of Bases
•Bases have pH values of above 7.
•A base which is water soluble is referred to as analkali.
•In basic (alkaline) conditions red litmus paper turnsblueand
methyl orange indicatorturnsyellow.
•Bases are substances which can neutralise an acid, forming
a salt and water.
•Bases are usuallyoxidesorhydroxidesof metals.
•When alkalis react, theygainelectrons to form
negative hydroxide ions (OH
–
).
•The presence of the OH
–
ions is what makes the aqueous
solution an alkali.
Example:Sodium Hydroxide
NaOH(s) → Na
+
(aq) + OH
–
(aq)

8. Monday, April 6, 2020Monday, April 6, 2020
Sir. Muhammad Abdul Mageid
8.1 The Characteristic Properties of Acids and Bases
Typical reactions of bases
2-Bases and acids
•When they react with an acid,a neutralisation reaction occurs.
•Acids and bases react together in a neutralisation reaction and produce a
salt and water:
Acid + Base → Salt + Water

9. Monday, April 6, 2020
Sir. Muhammad Abdul Mageid
8.1 The Characteristic Properties of Acids and Bases
2-Alkalis and ammonium salts
•Ammonium salts undergodecomposition when warmed with an alkali.
•Even though ammonia is itself a weak base, it is veryvolatileand can
easily by displaced from the salt by another alkali.
•A salt, water and ammonia are produced.
Example:
NH
4Cl+ NaOH→ NaCl+ H
2O + NH
3
•This reaction is used as a chemical test to confirm the presence of
the ammonium ion (NH
4
+
).
•Alkali is added to the substance with gentle warming followed by the test
for ammonia gas using damp red litmus paper.
•The litmus paper will turn fromredtoblueif ammonia is present.

10. Monday, April 6, 2020
Sir. Muhammad Abdul Mageid
8.1 The Characteristic Properties of Acids and Bases
Neutrality and Relative Acidity and Alkalinity
The pH scale
•The pH scale is a numerical scale which is used to show
howacidicoralkalinea solution is.
•It goes from 1 –14 (extremely acidic substances can have values of below 1).
•All acids have pH values ofbelow7, all alkalis have pH values ofabove7.
•Thelowerthe pH then themoreacidic the solution is.
•Thehigherthe pH then themorealkaline the solution is.
•A solution of pH 7 is described as beingneutrale.g. water.
The pH scale

11. Monday, April 6, 2020
Sir. Muhammad Abdul Mageid
8.1 The Characteristic Properties of Acids and Bases
2-Universal indicator
•Universal indicator is a mixture of
differentindicatorswhich is used
to measure the pH.
•A drop is added to the solution and
the colour is matched with a colour
chart which indicates the pH which
matches specific colours.
The pH scale with the Universal
Indicator colours which can be used to
determine the pH of a solution

12. Monday, April 6, 2020
Sir. Muhammad Abdul Mageid
8.1 The Characteristic Properties of Acids and Bases
The importance of pH and soil acidity
•Soil pH is analysedto indicate theacidityoralkalinityof soil.
•Most plants favoura pH value of between5.5and8.
•Changes in soil which cause a pH to be outside this range adversely affect
plant processes resulting in reducedgrowthandcropyield.
•Soils may become acid fromacid rain, overuse offertilizerswhich contain
ammonium salts or by the excessive breakdown of organic matter
by bacteria.
•Crushed or powdered limestone (calcium carbonate) or lime (calcium oxide)
or slaked lime (calcium hydroxide) is added to neutralise the excess acidity
in the soil.
•The addition process must be carefully monitored though, as if added in
excess, further damage could be done if the pH goes too high.

13. Monday, April 6, 2020
8.1 The Characteristic Properties of Acids and Bases
Proton Transfer and Weak and Strong Acids and Bases
Proton transfer in acids and bases (Alkalis)
Diagram Showing the Role of Acids and
Bases in the Transfer of Protons
Sir. Muhammad Abdul Mageid

14. Monday, April 6, 2020
8.1 The Characteristic Properties of Acids and Bases
Proton transfer in acids and bases (Alkalis)
Acids Bases (Alkalis)
Acids areproton donorsas
they ionize in solution producing
protons, H
+
ions.
These H
+
ions make the
aqueous solution acidic.
Bases (alkalis) areproton
acceptorsas they ionize in
solution producing OH
–
ions
which can accept protons.
These OH
–
ions make the
aqueous solution alkaline.
Sir. Muhammad Abdul Mageid

15. Monday, April 6, 2020
8.1 The Characteristic Properties of Acids and Bases
Strong acids and bases
Acids and alkalis can be
eitherstrongorweak, depending on how
many ions they produce when dissolved in
water.
Strong acids and basesionizecompletelyin
water, producing solutions ofvery low pHfor
an acid orveryhighpH for a base.
Strong acids include HCl, H
2SO
4and HNO
3
strong bases include the Group(I)hydroxides
NaOH, KOH , RbOH
Sir. Muhammad Abdul Mageid

16. Monday, April 6, 2020
8.1 The Characteristic Properties of Acids and Bases
Strong acids and bases
Sir. Muhammad Abdul Mageid

17. Monday, April 6, 2020
8.1 The Characteristic Properties of Acids and Bases
Sir. Muhammad Abdul Mageid
Weak acids and bases
Weak acids and basespartiallyionize in water and produce pH values
which are closer to themiddleof the pH scale.
Weak acids include organic acids such as ethanoicacid, CH
3COOH and
weak bases include aqueous ammonia NH
4OH.
For both weak acids and bases, there is usually an equilibrium set-up
between the molecules and their ions once they have been added to
water.
Example of a weak acid: Example for a weak base:
Propanoicacid
CH
3CH
2COOH ⇌H
+
+ CH
3CH
2COO
–
Aqueousammonia
NH
3+ H
2O ⇌NH
4
+
+ OH
–

18. Monday, April 6, 2020
8.1 The Characteristic Properties of Acids and Bases
Sir. Muhammad Abdul Mageid
Effect of concentration on strong and weak acids and alkalis
A concentrated solution of either an acid or a base is one that contains
ahigh numberof acid or basemoleculesper dm
3
of solution.
It does not necessarily mean that the acid or base is strong though, as it
may be made from a weak acid or base which does not dissociat
For example a dilute solution of HClwill be more acidic than a
concentrated solution of ethanoicacid, since most of the
HClmoleculesdissociatebut very few of the CH
3COOH do.
Exam Tip
In acid-base chemistry, the termsstrongandweakrefer to the ability to
dissociate and produce H
+
/OH
–
ions.
If referring to concentration when answering a question, then the
wordsconcentratedordiluteshould be used.