Standard Grade Chemistry
Summary Notes
Topic 8 : Acids And Alkalis
General
Learning Outcomes
1.
Metal oxides are basic.
2.
Most basic oxides are insoluble in water, but some dissolve to give alkalis.
3.
Non-metal oxides are acidic.
4.
Acidic oxides dissolve in water to give acids.
5.
Sulphur dioxide in water leads to acid rain.
6.
Acid rain is harmful to plants and animals. It can damage certain buildings,
especially those made from marble and limestone. It can also speed up the
corrosion of metals.
7.
Acids turn indicator solution red or orange and have pH numbers in the range
0-6.
8.
Alkalis turn indicator solution blue or purple and have pH numbers in the range
8-14.
9.
Water and other neutral solutions turn indicator solution lime green and have
a pH number equal to 7.
10.
Examples of acids in the lab ; sulphuric, hydrochloric and nitric acids.
11.
Examples of acids in the home ; ethanoic (in vinegar) and citric (in fruit)
12.
Examples of alkalis in the lab ; sodium, potassium and calcium hydroxides.
13.
Examples of alkalis in the home ; ammonia (household cleaners) and milk of
magnesia (for indigestion).
14.
Pure water is a covalent liquid but contains a small number of ions.
15.
Acid solutions contain hydrogen ions – H+ (aq)
16.
Alkalis contain hydroxide ions – OH-(aq)
17.
When acids are diluted, the concentration of H+ (aq) decreases and the pH
rises towards 7.
18.
When alkalis are diluted, the concentration of OH-(aq) decreases and the pH
decreases towards 7.
Credit
Learning Outcomes
1. Water is neutral because it contains an equal number of H+ (aq) and OH-(aq)
ions.
2. Water and all aqueous solutions contain H+ (aq) and OH-(aq) ions.
3. Acid solutions contain an excess of H+ (aq) ions.
4. Alkali solutions contain an excess of OH-(aq) ions.
In addition, you should be able to do the calculations involving :
(i)
the gram formula mass of substances
(ii)
the number of moles from the mass and vice versa
(iii)
moles, volume and concentration using :
moles
volume (l) conc (mol/l)
How Acid And Alkali Solutions Can be Made
(General)
Non-metal elements can react with oxygen to form compounds called non-metal
oxides.
Example :
sulphur + oxygen  sulphur dioxide
S (s)
O2(g)
SO2(g)
Non-metal oxides which dissolve in water produce acid solutions.
Example :
sulphur dioxide + water 
SO2(g)
+ H2O(l)
sulphurous acid
H2SO3(aq)
Alkaline solutions are made when soluble metal oxides or hydroxides dissolve in
water.
 Check the solubility table in your data book to see which oxides dissolve
Example :
sodium oxide + water  sodium hydroxide solution
Na2O(s) + H2O(l)  2NaOH(aq)
The pH Scale
(General)
The pH scale is a continuous range of numbers which indicate the acidity or
alkalinity of solutions.
 acids have a pH of less than 7
 alkalis have a pH of more than 7
 pure water and neutral solutions have a pH equal to 7
The chart below shows the pH of some common substances.
lemon juice
pH
1
2
3
4
vinegar
salt solution
5
6
7
8
water
9
ammonia solution
10
11
12
13
14
milk of magnesia
The Ions Present In Acid And Alkali Solutions
An acid solution is one which contains hydrogen ions – H+ (aq)
An alkaline solution is one which contains hydroxide ions – OH- (aq)
The concentration of ions in pure water is small.
(General)
(Credit)
In water and in neutral solutions the concentration of H (aq) and OH (aq) is the
same.
That is why water is neutral.
An acidic solution contains more H+ (aq) ions than does pure water.
An alkali solution contains more OH- (aq) ions than does pure water.
+
-
What Happens When An Acid And An Alkali Are Diluted?
(General)
Diluting an acid by adding more water increases the pH of the solution towards
7, making it less acidic.
Diluting an alkali by adding more water decreases the pH of the solution
towards 7, making it less alkaline.
Diluting an acidic solution decreases the concentration of H
causes the pH to increase towards 7.
+
(aq)
(Credit)
ions. This
Diluting an alkali solution decreases the concentration of OH- (aq) ions. This
causes the pH to decrease towards 7.