Grade 10 Physical Sciences

1. Sour taste in lemons is caused by citric acid.
Acids can ‘burn’ holes in metals or clothing.
Acids can damage skin & cause blindness in the eye.
Rain containing acid can very slowly dissolve cement,
concrete & stone buildings. H2SO4 reacting with material
1

2. The Arrhenius model
1. Acids are regarded as substances that
liberate H+ ions when dissolved in water.
Hydrochloric acid: HCl → H+ + Cl-
Nitric acid: HNO3 → H+ + NO3
-
Sulphuric acid: H2SO4 → 2H+ + SO4
2-
Phosphoric acid: H3PO4 → 3H+ + PO4
3-
Arrhenius acids and bases
2

3. 2. Bases (Ahrrenius model) are regarded as
substances that form OH- ions when
dissolved in water.
Sodium hydroxide: NaOH → Na+ + OH-
Potassium hydroxide: KOH → K+ + OH-
Calcium hydroxide: Ca(OH)2 → Ca2+ + 2OH-
Although both acids, bases & water contain
covalent molecules, the molecules are polar in
nature & the acids & bases dissolve in water to
form new ions in solution.
Ahrrenius acid & base
3

4. ∂+ ∂-
H Cl Cl
_
OH
H
Cl-
OH
H
+
H3O+
There is a polar covalent bond in the HCl molecule as
a result of the unequal sharing of the electron pair.
The bond in the HCl breaks and new Cl- and H3O+ ions
are formed.
These ions are then surrounded by the other water molecules.
H
Ionisation of acids
4

5. When an acid reacts with a base there is an exchange of
ions:
H2SO4 + Ca(OH)2 → CaSO4 + 2H2O
Expressed slightly differently – showing the ions in
each solution:
2H+ + SO4
2- + Ca2+ + 2OH- → CaSO4 + 2H2O
The - & + pairs in the solutions swop & we form a
salt & water.
This called neutralisation.
Ion exchange & other reactions
5

6. In the Lowry-Brønsted model, an acid is a proton donor
and a base is a proton acceptor.
This is the modern concept as to
what constitutes an acid and a base.
Besides the alkalis (metal hydroxides) there are many
other bases, such as metal oxides and metal
carbonates.
We thus need a wider definition of acids & bases.
Lowry Bronsted acids & bases
Arrhenius vs Lowry-Bronsted acids & bases
6

7. Acid: HCl → H+ + Cl-
Acid → proton + ion
Base: NH3 + H+ → NH4
+
base + proton → ion
An acid is ∴any substance that can donate a
proton when reacting.
A base is any substance that receives a proton
when reacting.
The reaction between an acid and a base is the
donating and receiving of a proton.
7

8. The following reaction illustrates an acid-base reaction:
HCl + NH3 → Cl- + NH4
+
Acid
donating
a proton
Base
receiving
a proton
Complete the following equations
& identify the acids, bases &
respective conjugates:
H2SO4 + CaCO3 →
NH3 + H2O →
HCn + NH3 →
HNO3 + NaOH → 8

9. An acid & a base that are related as a result of
a transfer of protons, is called a conjugate
acid –base pair.
• an acid forms a conjugate base
• a base forms a conjugate acid.
HCl(aq) + H2O(l)  Cl-(aq) + H3O+(aq)
Acid 1 base 2 base 1 acid 2
Conjugate acid-base pair 1
Conjugate base-acid pair 2
Conjugate acid base pairs
9

10. 10

11. Ampholytes are substances that can act as
either an acid or as a base, depending upon
what it is reacting with.
Water as an acid: H2O + NH3  NH4
+ + OH-
Water as an base: HNO3 + H2O  H3O+ + NO3
-
Other ampholytes: HSO4
-, HCO3
- & H2PO4
-.
Now work out the 2 equations each of these
ampholytes - to illustrate how they react as
ampholytes.
11

12. Whenever we add an acid & a alkali we always
produce a salt & water.
We say the alkali has neutralised the acid – or vice-
versa.
HCl + NaOH → NaCl + H2O
H2SO4 + Ca(OH)2 → CaSO4 + H2O
Acid + base → salt + water
In order to tell if the one has neutralised the other, we
need to use an indicator to see when there is a change
in colour..
Neutralisation reaction
12

13. An acid plus a alkali gives a salt and water
HCl + NaOH  NaCl + H2O.
An acid plus a metal oxide gives a salt and
water. HCl + Na2O  2NaCl + H2O
An acid plus a metal gives a salt and
hydrogen. 2HCl + Zn  ZnCl2 + H2
An acid plus a carbonate gives a salt, CO2
and water. 2HCl + Na2CO3  2NaCl + CO2 + H2O
Reactions of acids and bases 13

14. Indicators are organic dyes that have a particular
colour in an acid and a different colour in an alkali
(soluble base).
The indicator litmus is red in an acid and blue in an
alkali. This can be in a liquid or paper form.
HCl NaOH
Add litmus Indicators
14

15. Name
of indicator
Colour
acid
Colour
base
pH
range
Methyl Orange Red Yellow 3 - 4
Bromothymol Blue Yellow Blue 6 - 8
Phenolphthalein Clear Red 8 - 10
Indicators
Natural indicators can be
produced from red cabbage, red
lettuce, red flower petals, red
wine, red grape juice & diluted
beetroot juice.
Make an indicator
15

16. Universal indicator – in liquid or paper form, is
able to change to a whole range of colours,
thus indicating the degree of acidity or
alkalinity. The closer to red – the more acidic.
Universal indicator
16

17. Many salts can be produced from acid base
reactions:
H2SO4 + Na2O  Na2SO4 + H2O
H2SO4 + CuO  CuSO4 + H2O
H2CO3 + Ca(OH)2  CaCO3 + 2H2O
If precipitates are formed – filter.
If ions remain in solution – evaporate
water off.
17

18. To establish whether an alkali has neutralised an
acid, add the indicator to the acid.
Now slowly add the alkali – when the indicator
changes colour – you have neutralised the acid &
have thus added enough alkali.
Burette
Acid with
litmus
Add alkali
As soon as the indicator changes
colour you have added enough
alkali to just neutralise the acid.
Adding more alkali just makes
the solution more alkaline & the
indicator will go to a darker blue.
Energy is released in process.
Titration
18

19. Doing a titration 19

20. mA
milli-ammeter
Carbon
rods
Pour different liquids into a beaker
and then connect up the electrical
circuit as in sketch.
Since all acids & alkalis contain ions, they will
conduct electricity & there should be a reading every
time you use an acid or an alkali.
However the ability to conduct an electric current will
differ quite a lot and depends upon the ability of the
substance to form ions in solution.
Conductivity of acids & bases
20