Acids and Metals – Revision
The pH scale
The pH scale is a numbered scale going from 1 to 14 which tells us whether a solution
is acid, neutral or alkaline.
To test pH we can use indicator papers or solutions and the colour they turn is
compared with a chart which gives the pH number.
The pH scale is shown below for one type of indicator
Colour of
indicator
pH
Acid, Alkali or
Neutral
Very deep red
deep red
red
orange red
Orange
Yellow
1
2
3
4
5
6
Acid
Acid
Acid
Acid
Acid
Acid
Green
7
Neutral
Blue-green
Pale blue
Deep blue
Faint purple
pale purple
purple
Deep Purple
8
9
10
11
12
13
14
Alkali
Alkali
Alkali
Alkali
Alkali
Alkali
Alkali
Acids pH < 7
Neutral pH = 7
Alkali pH > 7
We tested common substances and found fruit juices and vinegar to be acids.
Water and salt water were neutral, having pH =7
Toothpaste, indigestion powder and baking soda were alkaline.
Diluting Acids and Alkalis
We took an acid and measured its pH which was low. We then diluted it gradually
with water. We found that the pH rose gradually and eventually reached 7. By
diluting it we were making it less acid.
When we diluted an alkali we found its pH was high at the start but as we added more
and more water the pH fell until it eventually reached 7. By diluting it we were
making it less alkaline.
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Acids and Metals – Revision
Neutralisation
We carried out a number of experiments where we added acid gradually to alkali with
Universal indicator present so that the pH could be checked.
We found that the acid gradually cancelled out or neutralised the alkali.
We did the same experiment in reverse adding the alkali to the acid and found that the
alkali neutralised the acid.
When we take an indigestion tablet we are trying to cure a problem caused by too
much acid in our stomachs. The indigestion tablet contains an alkali which gradually
cancels out or neutralises some of the acid in our stomach making us more
comfortable.
Bee stings are alkaline and can be relieved by using vinegar which is an acid to
neutralise the alkali.
The Reactivity Series
We carried out a number of experiments with metals and found that some are more
reactive than others. When the metals are put in a league table in order of reactivity it
looks like this:
Potassium
Sodium
Calcium
Magnesium
Aluminium
Zinc
Iron
Nickel
Tin
Lead
Copper
Mercury
Silver
Gold
Platinum
Most reactive – react easily with cold water.
The MAZINTL metals react with dilute acid
Very unreactive – won’t even react with acid
Uses of Metals
Un-reactive metals like copper are suitable for use as water pipes because they won’t
react with or dissolve in the water.
Gold and platinum do not react with air, water or chemicals in the home so can be
used as jewellery. They keep a nice clean shiny appearance.
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Acids and Metals – Revision
Getting Metals from the Earth
Since gold is un-reactive it can be found as the pure metal in the earth – it does not
react with its surroundings like potassium, calcium and magnesium would.
Iron is found on the earth in a substance called iron ore. To get the iron from the ore
takes a lot of heat energy. Iron is separated from its ore in a furnace called the Blast
Furnace. It is heated with coke and limestone.
Molten iron is produced as well as a waste product called slag.
Potassium, sodium and calcium are very reactive metals and are among the latest
metals to be discovered. Only relatively recently has man had the technology to get
these metal out of their ores.
Displacement
We carried out some experiments in which a reactive metal reacted with a solution
containing another metal.
For example magnesium reacted with blue copper sulphate solution. The solution
became gradually less blue and small pieces of copper appeared at the bottom. The
magnesium began to dissolve.
The reactive magnesium had displaced (pushed out) the less reactive copper from its
solution.
A metal can displace a metal lower in the Reactivity Series from its compound.
Electrolysis
We passed an electric current
through copper chloride
solution (CuCl2).
Copper metal stuck to the
negative rod and bubbles of
chlorine gas (smell of bleach)
appeared at the positive rod.
The electrical energy had
broken the compound to give
the metal
Electroplating
We used electricity to cover a piece of copper in nickel. This process is called
electroplating. It can be used to improve the appearance of cheaper metals by plating
them with precious metals.
This means that gold plated jewellery is more affordable than jewellery containing
mostly gold.
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