
2. pH scale, measuring pH,
what is an acid?,
what is a base?, what is a salt?
neutralisation - salt formation,
simplified
ionic theory of acids & alkalis
Index of all my GCSE notes on acids, bases
and salts
All my
GCSE Chemistry Revision
notes
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This page introduces and explains the pH scale measuring the
relative acidity and alkalinity of aqueous solutions, that is solutions of
substances dissolved in water. The use of indicators is described and several
well known indicators are tabulated showing their different colours in solutions
of different pH. The ionic theory of acids, bases and neutralisation is simply
described and why explains why solutions are either acid, neutral or alkaline.
These revision notes on the pH scale and the ionic theory of neutralisation of
acids and alkalis should prove useful for the new AQA chemistry, Edexcel
chemistry & OCR chemistry GCSE (9–1, 9-5 & 5-1) science courses.
Doc Brown's
chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE chemistry, O level
& ~US grades 8, 9, 10 school science courses for ~14-16 year old science
students for national examinations in chemistry topics including acids
bases alkalis salts preparations reactions
2.
The pH scale, indicators acids,
alkalis (bases), neutralisation & ionic theory
2a.
Introduction to
the pH scale What is the pH scale?
The
colours observed in solutions when universal indicator is added
-
The pH scale is a
measure of the relative acidity or alkalinity of a solution
(see diagram).
-
So, knowing the pH of a solution, you know how acid or
alkaline it is by reference to the pH scale (diagram above) or whether the
solution is neutral.
-
The smaller the pH number, the more acid it is, the greater
the pH number, the more alkaline it is, and if the pH is close to 7, you
have a more or less neutral solution that has neither acidic or alkaline
chemical properties.
-
Lots of examples of solution pH values are tabulated with everyday examples of
acid/alkaline chemistry are described in
section 1.
-
pH can be approximately measured using
indicator solution by putting a few drops of universal indicator
into a solution and comparing the colour formed with a standard chart
(picture above). -
You can also used paper impregnated with an indicator
solution (pH paper), the paper is dipped in the solution and again the colour
matched with a pH chart.
-
This is quite handy for testing soil
mixed and shaken with water.
-
You can get special soil testing kits which
use indicator solution and the colour of the indicator in the water is
matched with a chart after the soil has settled out.
-
pH can be very accurately measured with a special
instrument called a pH meter using a glass electrode probe which is
calibrated with standard buffer solutions of accurately known pH (see
photographs and note at the end of the page).
WHAT IS AN INDICATOR?
An indicator is a
dye substance or mixture of coloured substances that when added to the solution gives a
different colour depending on the pH of the solution.
-
Universal indicator
solution or paper, is prepared from mixing several indicators to give a
variety of colours to match a wide range of pH values from very acid to
very alkaline.
-
The mixture of dyes responds to
changes in pH, so depending on what the pH is, i.e. how acid, how alkaline
or neutral the solution is, the indicator tells which it is.
-
Not only that, an indicator like
universal indicator' can tell you how strongly acid or strongly alkaline the
solution is by giving you the pH to about the nearest indicator.
-
A calibrated instrument called a
pH meter can give the pH to two decimal places.
-
It is a
very handy indicator for showing whether the solution is very
weakly/strongly acidic (pH <7) or
alkaline (pH > 7) or
neutral (pH = 7) and gives the pH to the nearest pH unit.
-
-
The diagram above gives the sort
of range of colours you get from using universal indicator, which is
a complex mixture of different dye molecules that respond to changes in pH.
Theoretically there is no
limit to the pH scale, but most solutions are between pH 0 and pH
14.
-
For example, looking at the 'extremes', 1M hydrochloric acid (HCl) has a pH of 0 and 10M
HCl has a pH of –1 and these would be described as strongly acidic
solutions.
-
1M sodium hydroxide (NaOH) has a pH of 14, but 10M
potassium hydroxide (KOH) has a pH of 15 and these would be described as
strongly alkaline solutions.
-
The closer the pH is to 7, the
less strong is the acid or alkali.
-
However the solubility limits of
substances in water ensures that its almost impossible to get below –1 or
above 15 and most laboratory measurements will be in the range pH 1 to pH
14 .
Note
1: M is the old
shorthand for solubility in mol/litre or mol dm–3.
Note 2:
The pH scale is known as a logarithmic scale of base 10.
Other common indicators used
in the laboratory
often
used in titrations – e.g. salt preparation (a)
Indicator |
colour
in acid pH<7 |
colour
in neutral pH=7 |
colour
in alkali pH >7 |
litmus |
red |
'purple' |
blue |
phenolphthalein* |
colourless |
colourless |
>9 pink |
methyl
orange* |
<3.5 red,
orange
about pH 5, > 6 yellow |
yellow |
yellow |
methyl red* |
<5 red,
orange, >6 yellow |
yellow |
yellow |
bromothymol
blue* |
<6 yellow |
green |
>8 blue |
*
Used in titrations - see section 10,
litmus is not a good indicator for use in titrations.
Despite the wide use and convenience of indicators the most accurate
way of measuring pH is electronically using a special pH meter -
illustrated below.
 |
 |
You can measure the pH
of a solution very accurately using a pH meter and a glass membrane pH
probe. The pH meter is calibrated against
a standard buffer solution of very accurately known pH |
2b. Introduction to
Acid–Base (including Alkalis) Theory including Neutralisation
-
Water is a neutral liquid
with a pH of 7 (green with universal indicator).
-
When a substance
dissolves in water it forms an aqueous (aq) solution that may be acidic,
neutral or alkaline.
-
Acidic solutions have a pH
of less than 7, and the lower the number, the stronger the acid it, or
the more acidic the solution.
-
An acid is a substance that reacts
with metals and bases to form a salt.
-
In more advanced theory an acid is
defined as a proton donor ...
-
The
colour can range from orange–yellow (pH 3–6) for partially ionised weak acids like ethanoic acid (vinegar) and carbonated water.
-
Strong acids
like hydrochloric, sulfuric and nitric are fully ionised and give a pH 1 or less and a
red
colour with universal indicator
or litmus paper.
-
Neutral solutions have a
pH of 7.
These are quite often solutions of salts, which are
themselves formed from neutralising acids and bases.
-
The 'opposite' of an acid
is called a base.
-
A base is a chemical that reacts with
acids to form a salt e.g. a metal oxide, hydroxide or carbonate.
-
In more advanced theory a base is
defined as a proton acceptor ...
-
Alkaline solutions have a
pH of over 7
and the higher the pH the stronger is the alkali, the more alkaline is
the solution.
-
Weak alkalis (soluble bases)
like ammonia give a pH of 10–11 but strong alkalis (soluble bases) like
sodium hydroxide give a pH of 13–14.
-
Alkalis
give blue–purple–violet colour with universal indicator or litmus paper.
-
NEUTRALISATION usually
involves mixing an acid (pH <7) with a
base
or alkali (pH > 7) which react to form a
neutral
SALT solution of pH ~7
2c. A bit of advanced
Acid–Base Theory
In acid solutions
there are more H+ ions than OH– ions,
so an excess of hydrogen ions makes the solution acidic with a pH of
less than 7.
In alkali solutions
there are more OH– ions than H+ ions ,
so an excess of hydroxide ions makes the solution an alkaline with a
pH of over 7.
When alkalis and acids
react, the 'general word' or 'molecular formula' equation might be
for NEUTRALISATION ...
So the 'acidic'
hydrogen ions cancel out the 'alkaline' hydroxide ions by
combining to form neutral water, AND give a neutral solution of
a salt.
and, in this case,
the remaining ions e.g. sodium Na+(aq) and
chloride Cl–(aq)
become the salt crystals of sodium chloride NaCl(s)
on evaporating the
water.
-
So the salt is formed
from the residual ions when all the hydrogen ions and hydroxide ions
have reacted.
-
In this simple case the
sodium ions and chloride ions don't take part in the reaction and
are known as spectator ions.
-
BUT, on evaporation of
the solution, the sodium ions and chloride ions will come together
and crystallise out of solution as the 'salt' sodium chloride.
BASES e.g. oxides,
hydroxides and carbonates, are substances that react and neutralise
acids to form salts and water.
or
Ca(OH)2 calcium
hydroxide.
Bases which are water
insoluble include CuO copper(II) oxide, MgO magnesium oxide
and these will also react and dissolve in acids to form salt
solutions e.g.
-
ACID +
BASE ==>
SALT +
WATER
-
copper oxide +
sulfuric acid ==> copper sulfate + water
-
H2SO4(aq)
+ CuO(s)
==> CuSO4(aq)
+ H2O(l)
After
a neutralisation, the
salt
solutions
consist of a mixture of positive and negative
ions (and their names are in the salt name!) e.g.
-
sodium chloride (NaCl) is a mixture of Na+
and Cl– ions in the ratio 1:1 (from hydrochloric acid and
sodium hydroxide)
-
calcium chloride (CaCl2)
is a mix of Ca2+ and Cl– ions of ratio 1:2 (from
hydrochloric acid and calcium oxide/hydroxide)
-
magnesium nitrate (Mg(NO3)2) is a mix of Mg2+
and NO3– ions in the ratio 1:2 (from nitric acid
and magnesium oxide/hydroxide)
-
aluminium sulfate (Al2(SO4)3)
consists of Al3+ and SO42–
ions in the ratio 2:3 (from sulfuric acid and aluminium
oxide/hydroxide)
Important NOTE on the pH Scale
(i) pH is a measure of the hydrogen ion (H+)
concentration
The lower the pH, the higher the
hydrogen ion concentration, the more acid the solution.
I know this seems confusing, but that's the way the pH
scale has been defined historically.
(ii) Each pH unit change is equivalent to a 10x change in
concentration of the hydrogen ion
For example changing the pH of a
solution from pH 4 to pH 3 makes the solution 10x more acidic.
Changing a solution's pH from 4 to 6 makes it 100x
less acidic (10 x 10).
Simple neutralisation experiments
-
(1) Starting with 25-50 cm3 of
a dilute alkali solution (e.g. sodium hydroxide), add some universal
indicator solution (blue-violet in excess alkali).
-
Add slowly in small
portions, until in excess, a dilute acid solution of similar concentration
(e.g. hydrochloric acid).
-
The colour will change from blue to green
(neutral) to red (excess acid), with other colours in between.
-
You can
follow the rise in pH with a colour chart, or you use universal indicator
paper or a pH meter instead of the universal indicator solution.
-
You should find that the pH falls from
~pH 13 to pH 7 (solution neutralised) and then falling further to ~pH 1 with
excess acid.
-
If you plot a graph it should roughly
look like the
red line right diagram by adding acid to the alkali as
the pH falls.
-
You can do this experiment very
accurately using a pipette, burette and pH meter.
-
You can also reverse the experiment by
adding the
alkali to the acid and produce the blue graph line and see
the pH rise.
-
(2) Starting with 25-50 cm3 of
a dilute hydrochloric acid solution, add powdered calcium oxide (lime) or
calcium hydroxide (slaked lime) in small portions at a time.
-
Again, follow
the pH as it rises from pH 0-1 to pH 7 (neutralised) and then rising further to ~pH
13
with excess of the calcium oxide/hydroxide alkali.
-
If you plot a graph it should roughly
look like the blue line (above right diagram).
See section 7.
for more on
Changes in pH in a
neutralisation reaction, choice and use of indicators
GCSE/IGCSE chemistry higher level notes for the
advanced proton/hydrogen ion theory of acids and bases
TOP OF PAGE
GCSE/IGCSE Acid & Alkalis revision notes sub–index:
Index of all pH, Acids, Alkalis, Salts Notes 1.
Examples of everyday acids, alkalis, salts, pH of
solution, hazard warning signs : 2.
pH scale, indicators, ionic theory of acids–alkali neutralisation : 4.
Reactions of acids with
metals/oxides/hydroxides/carbonates, neutralisation reactions : 5.
Reactions of bases–alkalis
like ammonia & sodium hydroxide : 6. Four methods
of making salts : 7. Changes in pH in a
neutralisation, choice and use of indicators : 8. Important formulae
of compounds, salt solubility and water of crystallisation :
10.
More on Acid–Base Theory and Weak and Strong Acids
See also
Advanced Level Chemistry Students Acid–Base Revision
Notes – use index
Multiple choice revision quizzes and other worksheets
-
GCSE/IGCSE foundation–easier multiple choice quiz on pH, Indicators, Acids,
Bases, Neutralisation and Salts
-
GCSE/IGCSE higher–harder multiple choice quiz on pH, Indicators, Acids,
Bases, Neutralisation and Salts
-
GCSE/IGCSE Structured question worksheet on Acid
Reaction word equations and
symbol
equation questions
-
Word
equation answers and
symbol
equation answers)
-
GCSE/IGCSE word–fill worksheet on Acids,
Bases, Neutralisation and Salts
-
GCSE/IGCSE
matching pair quiz on Acids, Bases, Salts and pH
-
See also
Advanced Level Chemistry Students Acid–Base Revision
Notes – use index
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