Chemistry: 5. Solutions and Crystals
Syllabus
OC15 Investigate the solubility of a variety of substances in water and the effect of temperature on solubility
OC16 Examine the difference between a dilute, concentrated and saturated solution
OC17 Grow crystals using alum or copper sulfate
Student Notes
A solution is formed when a solid dissolves in a liquid
The solid is referred to as the solute.
The liquid is referred to as the solvent.
A dilute solution is one where there is a lot of solvent and only a little solute
A concentrated solution is one where there is a lot of solute and only a little solvent
A solution can be made more concentrated by adding more of the solute.
A saturated solution is one where no more solute will dissolve in the solvent
Crystals
Some solids are always found in the form of a crystal.
Crystalline solids have definite shapes because the atoms in the crystal are in a regular
pattern.
Examples include sugar and salt diamonds and ice.
Salt is composed of two different types of atom, sodium (symbol Na) and chlorine
(symbol Cl), arranged in an orderly pattern as shown.
Other solids are said to be non-crystalline.
Example: flour
Experiment: To grow crystals using copper sulphate
Procedure:
Keep adding copper to hot water until no more will dissolve
Allow to cool slowly.
Result:
Crystals begin to grow
1
Exam Questions
1. [2007]
Distinguish between a concentrated and a dilute solution?
2. [2007]
(i) Name a substance, other than water, that forms crystals.
(ii) Give one difference between crystalline and non-crystalline solids.
3. [2006 OL]
Describe how you could carry out an experiment to grow crystals using alum or copper sulphate.
Include a diagram of any equipment used.
4. [2009 OL]
In a school laboratory, a student investigated the solubility of a salt in water.
The amount of salt which dissolved in water at different
Temperature
temperatures was measured. The data collected is
°C
presented in the table below.
Solubility
g per 100 cm3 of water
(i) Use this data to draw a graph of solubility (y-axis)
against temperature (x-axis) using the grid provided below.
(ii) Use the graph to estimate the solubility at 60 °C.
(iii) What can you conclude about the solubility of the salt in water from the graph?
2
20
30
40
70
90
10
20
30
60
80
5. [2007]
A pupil investigated the effect of temperature on the solubility of the salt ammonium chloride in water. She
determined the maximum mass, in grams, of the salt that would dissolve in 100 g of water at various temperatures.
The data from this experiment are given in the table.
Solubility (g / 100 g water)
Temperature (0C)
29
0
37
20
46
40
55
60
66
80
77
100
(i) Plot a graph of solubility against temperature.
(ii) Use the graph to estimate the solubility of ammonium chloride at 70 oC.
(iii) What conclusion about the solubility of ammonium chloride can be drawn from analysis of the graph?
3
6. [2008]
The limit of solubility (maximum solubility) of oxygen gas (O2) in water was measured, in mg of oxygen per 100
g of water, at a number of different temperatures. These measurements are given in the table.
Solubility (mg / 100 g water)
Temperature (0C)
7.0
0
4.3
20
3.0
40
2.3
60
1.4
80
0.8
90
(i) Draw a graph of solubility (y-axis) against temperature (x-axis) in the grid provided below.
(ii) Use the graph to estimate the solubility of oxygen at 30 °C.
(iii) What effect has temperature on the solubility of oxygen in water?
(iv) Global warming has many implications. What implication, which could be inferred (concluded) from the
information in the graph, might global warming have for animals that live in water e.g. fish?
4
0.0
100
Other Test Questions
1. What is a solution?
2. Describe briefly, with the aid of a diagram, how you would investigate the effect of temperature on the
solubility of copper sulphate in water.
3. What is a physical change?
4. What is a chemical change?
5. What is a solvent?
5
Chemistry: 6. Water
Syllabus
OC14 Use cobalt chloride paper or anhydrous copper sulfate to test for water
OC30 Conduct a qualitative experiment to detect the presence of dissolved solids in water samples, and test water
for hardness (soap test)
OC31 Understand that some dissolved compounds, including compounds of calcium, cause hardness in water, and
that water hardness can be removed using an ion-exchanger
OC33 Describe the processes involved in the treatment of water supplied to domestic consumers
OC34 Investigate the de-composition of water by electrolysis; recall the composition of water
Student Notes
Tests for water
1. Water turns blue cobalt-chloride paper pink
2. Water turns anhydrous copper sulphate from white to blue.
To investigate the solubility of a variety of substances in water and the effect of temperature on solubility
Example; table salt
Determine the maximum mass, in grams, of salt that will dissolve in 100 g of water at various temperatures.
Plot a graph of the results.
To detect the presence of dissolved solids in water samples
Pour a water sample into a beaker and boil allow the water to boil off.
The dissolved solids will remain on the sides of the glass and on the bottom of the beaker.
Water hardness
Hard water does not easily form lather with soap
Hardness in water is caused by dissolved compounds, particularly compounds of calcium and magnesium
To test water for hardness
Hard water causes scum and requires a large volume of soap to form a lather, so to check for hard water simply add
soap and see if it forms a lather.
Advantages of hard water
Good source of calcium for strong bones and
teeth
Most prefer the taste of hard water
Disadvantages of hard water
Lime scale blocks pipes and reduces
efficiency of kettles, washing machines.
Wastes soap and leaves a scum which can be
difficult to remove from clothing.
6
Removal of hardness
Water hardness can be removed using an ion-exchanger
The ion exchanger works by replacing the ions that cause hardness with ions that don’t cause hardness e.g. sodium
ions
Soft water easily forms lather with soap
Water treatment
Water is treated at a waterworks to make it fit to drink.
There are five stages:
1. Screening – removes large debris by passing water through a wire mesh
2. Settling – aluminium sulphate is added to the water. It makes the particles stick together and settle on the bottom.
3. Filtration –very small dirt particles are removed through filtration
4. Chlorination – Chlorine added to kill germs (‘sterilising’/ ‘disinfecting’)
5. Fluoridation – In Ireland fluorine is added to the water to prevent tooth decay.
(Make up your own acronym for the five stages)
The composition of water
1. Water is composed of hydrogen and oxygen
2. There are two parts hydrogen for every one part oxygen
3. The chemical symbol for water is H2O
Electrolysis of water
Electrolysis is the splitting up of a compound by passing electricity through it
Electrolysis of water splits water into hydrogen and oxygen
To investigate the de-composition of water by electrolysis
1. Set up as shown (the apparatus is known as a Hoffman Voltameter)
Pure water is a poor conductor of electricity so a little dilute sulphuric
acid can be added.
2. Notice that the gases formed are in the ratio 2:1
3. Test for Hydrogen (it burns with a ‘pop’).
4. Test for oxygen (it relights a glowing splint).
7
Exam Questions
1. [2006 OL]
Water is essential for life and is composed of two elements.
(i) Name one of the elements that make up water.
(ii) Name a chemical that can be used to test for the presence of water.
Water Hardness
2. [2008 OL]
The diagram shows an apparatus set up to investigate water hardness.
One test tube has hard water while the other has soft water.
Study the diagram and answer the questions which follow.
(i) Why is it necessary to use the same amount of water in each test tube and to add the same
volume of soap solution to each test tube?
(ii) When both tubes were shaken a lather formed in test tube A but not in test tube B.
What does this tell you about the water in test tube A?
Chlorination
Fluoridation
Settling
Screening
(iii) Name an element whose compounds contribute to hardness in water.
3. [2009]
(i) State how to test water to confirm the presence of hardness?
(ii) Name a metallic element some of whose compounds cause hardness in water.
(iii) Give one effect of hard water.
4. [2007]
Flask A contains hard water. Some of this water was poured into the tube containing an ion
exchange resin.
The water that passed through the ion exchange resin was collected in flask B.
(i) Describe a test that you could perform on water samples from flask A and from flask B to
compare their hardness?
(ii) What result would you expect from this test?
(iii) What causes hardness in water?
Water Treatment
5. [2008 OL]
Water supplied to domestic consumers is treated.
(i) In the table write the letter R beside the name of the treatment used to remove large floating debris from the water.
(ii) In the table write the letter T beside the treatment used to help prevent tooth decay.
6. [2007]
Water supplied to domestic consumers has undergone five or more different processes in a water treatment plant.
Name one of the processes carried out on water in a treatment plant.
Give a reason why the treatment that you have named is carried out.
Electrolysis
7. [2006 OL]
The diagram shows an arrangement of apparatus suitable for the electrolysis of acidified
water.
Name the gas produced at the electrode X and state a test
for this gas.
8. [2006]
The diagram shows the electrolysis of water.
(i) Why is some acid added to the water?
(ii) Give a test for gas A.
(iii) The volume of gas A is twice that of gas B.
What does this tell us about the composition of water?
8
9. [2009 OL]
The apparatus on the right can be used to decompose water by electrolysis.
Acid is added to the water to allow an electric current to flow through the
water.
Answer the following questions about the electrolysis of water.
(i) Hydrogen gas is collected at A. What test could you carry out in the laboratory
to show that this gas is hydrogen?
(ii) Name the gas collected at B.
Other Test Questions
1. What is water made from?
2. Give four characteristics of water.
3. Describe a laboratory experiment to detect the presence of dissolved solids in water samples.
4. Give one advantage and one disadvantage of hard water.
5. (i) Describe with the aid of a labelled diagram how you would compare three different water sources for hardness.
(ii) List two variables involved in an investigation to test for water hardness:
(iii) What variable would you change and what variable would you expect to change a result:
(iv) How would you ensure that the investigation was fair?
6. What dissolved compounds cause hardness in water?
7. Name one method used to remove ‘hardness’ from water.
8.
(i) How would you test a water sample to see if it was hard or soft?
(ii) Name a substance used to remove the hardness from water:
(iii) Give one cause of hardness in water:
(iv) Explain the term ‘hard water’.
9. The following results were obtained from an experiment to test for hardness by adding soap flakes to three
different water samples.
Water Sample
Number of soap flakes
A
14
B
1
C
7
(i) Which sample is most likely from a limestone area?
(ii) Which sample is most likely distilled water?
(iii) How would a student ensure this was a fair test?
(iv) Give the name or formula of a substance that causes hardness in water.
10. Fill in the missing stages of the treatment of water and state the function of each of the five stages.
Screening =>
=> Filtration =>
=> Fluoridation
11. Describe the processes involved in the treatment of water supplied to domestic consumers.
12.
(i) Name the apparatus used to split water into its constituent parts.
(ii) Describe an experiment to investigate the de-composition of water by electrolysis.
(iii) What happens to water during electrolysis?
9
13.
(i) What is the process to split water into its two elements called?
(ii) Two gases are produced in this process, Name them:
(iii) Why is there twice as much of one of these gases produced?
(iv) Why is a small amount of sulphuric acid added to the water at the beginning of the experiment?
(v) What material is used to make the electrodes used in this electrolysis of water?
(vi) Name the other liquid added to water during this experiment.
10
Chemistry: 7. Ionic and Covalent Bonding
Syllabus
OC42 Recall that ionic bonding is an attraction between positive and negative ions; describe the bonding in NaCl
and MgO as examples
OC43 State what a molecule is, understand that covalent bonds involve the sharing of pairs of electrons, and describe
the bonding in H2, O2, H2O, CH4 as examples of covalent bonding
OC44 Investigate the ability of ionic and covalent substances to conduct electricity
Molecules
A molecule is made up of two or more atoms chemically combined
Basically a molecule is like a very small group of atoms that go around together.
Hydrogen usually goes around in pairs of hydrogen atoms, so we say that H2 is a Hydrogen molecule.
A water molecule is made up of one atom of oxygen together with two atoms of hydrogen; that’s why it has the
symbol H2O.
Bonding
Bonding occurs because all atoms try to have a full outer shell, and will lose, gain or share electrons in order to do so
There are two separate methods by which atoms join up:
1. Ionic bonding occurs when an atom loses or gains electrons
2. Covalent bonding occurs when an atom shares electrons.
Ionic bonding
An ion is a charged atom – i.e. an atom which has lost or gained electrons
If an atom loses an electron then it becomes a positive ion.
If an atom gains an electron then it becomes a negative ion.
In ionic bonding positive ions are attracted to negative ions.
An ionic bond is a bond formed by the force of attraction between two oppositely charged ions
An example of an ionic bond is sodium chloride
The bonds in sodium chloride are formed by sodium atoms losing electrons and chlorine atoms gaining electrons.
Examples of ionic bonding: (i) Sodium Chloride (NaCl)
The atomic number of sodium is 11.
It has an electronic configuration of 2, 8, 1. This means that it needs to lose the one electron which it has in its outer
shell in order to have a complete outer shell.
The atomic number of chlorine is 17.
Chlorine has an electronic configuration of 2, 8, 7. This means it needs to gain one
electron in order to have a complete outer shell.
When a sodium atom bonds with a chlorine atom the sodium atom loses its outer
electron to form a positive ion while the chlorine atom gains an electron to form a
negative ion.
The two atoms now have opposite charges and because opposite charges attract
both atoms move off together as a sodium-chloride (NaCl) molecule.
11
Examples of ionic bonding: (ii) Magnesium Oxide (MgO)
The atomic number of magnesium is 12.
It has an electronic configuration of 2, 8, 2. This means that it needs to lose the two
electrons which it has in its outer shell in order to have a complete outer shell.
The atomic number of oxygen is 8.
Oxygen has an electronic configuration of 2, 6. This means it needs to gain two
electrons in order to have a complete outer shell.
When a magnesium atom bonds with an oxygen atom the sodium atom loses its two
outer electrons to form a positive ion while the oxygen atom gains two electrons to
form a negative ion.
The two atoms now have opposite charges and because opposite charges attract
both atoms move off together as a magnesium-oxide (MgO) molecule.
Covalent bonding
A covalent bond is a bond formed by different atoms sharing electrons in order to have a complete outer shell
Examples of covalent bonds are H2, O2, H2O and CH4 (methane) molecules.
The bonds in these molecules are formed by the atoms in the molecules sharing electrons with each other..
Examples of covalent bonding: (i) A hydrogen molecule (H2)
The atomic number of hydrogen is 1.
It needs to have two electrons in its outer shell, and so if it ‘bonds’ with another
hydrogen atom and they both ‘share’ their electron with each other, it has the effect
of allowing both atoms to have a complete outer shell.
Examples of covalent bonding: (ii) An oxygen molecule (O2)
The atomic number of oxygen is 8.
It has an electronic configuration of 2, 6.
It needs to have 8 electrons in its outer shell, and so if it ‘bonds’ with another oxygen
atom and they both ‘share’ two of their electrons with each other, it has the effect of
allowing both atoms to have a complete outer shell.
Examples of covalent bonding: (iii) A water molecule (H2O)
The atomic number of hydrogen is 1 so it has 1 electron in its outer shell.
The atomic number of oxygen is 8 so it has 6 electrons in its outer shell.
Oxygen can share 1 electron with one hydrogen atom and a second electron with a
second hydrogen atom, which has the effect of allowing all atoms to have a
complete outer shell.
Examples of covalent bonding: (iv) A methane molecule (CH4)
The atomic number of hydrogen is 1 so it has 1 electron in its outer shell.
The atomic number of carbon is 6 so it has 4 electrons in its outer shell.
Carbon can share 1 electron with each of 4 different hydrogen atoms, which has the
effect of allowing all atoms to have a complete outer shell.
12
To investigate the ability of ionic and covalent substances to conduct electricity




Set up a circuit as shown in the
diagram.
Select a number of different
substances for testing.
If the substances are solid then they
should be dissolved in pure water
before testing.
If the bulb glows then the substance
is a conductor and if it doesn’t glow
then it is an insulator.
Why were the solid substances dissolved
in water before the investigation?
Answer:
To allow the particles to move
13
Exam Questions
Ionic Bonding
1. [2007 OL]
Complete the sentence: In _________ bonding positive ions are attracted to negative ions.
2. [2006 OL]
The bonds in sodium chloride are formed by sodium atoms losing electrons and chlorine atoms gaining electrons.
Name the type of bond found in a sodium-chloride crystal.
3. [2008]
Atoms of different elements can form compounds by bonding together.
What is an ionic bond?
4. [2008]
A pupil investigated the ability of covalent
and ionic substances to conduct electricity.
Four substances were selected. One was a
liquid. The other three substances were
solids and these were dissolved in pure
water before testing.
The apparatus used in the investigation is
drawn below. When the liquids were tested
the bulb did not glow in some cases (Liquid
type A) and the bulb glowed in other cases
(Liquid type B).
The results of the investigation are given in
the table.
Liquid
Liquid type
Cooking oil
A
Table salt
B
Table sugar
A
Copper sulphate
B
(i) Name the ionic substances in the table. Give a reason for your answer.
(ii) Three of the substances tested are solid at room temperature. Why were these substances dissolved in water before
the investigation?
5. [2009]
The diagram shows sodium ions (+) and chloride ions (-) in part of a crystal of
table salt, sodium chloride.
(i) How are sodium ions and chloride ions formed from their atoms?
(ii) What force holds the ions together in sodium chloride?
(iii) Name one other compound that is composed of ions.
Covalent Bonding
6. [2007 OL]
Complete the sentence: In _________ bonding pairs of electrons are shared.
7. [2007]
The diagram shows a molecule of C60. It has 60 carbon atoms covalently bonded together.
This molecule is nick-named the ‘Buckey Ball’.
Explain the underlined term.
8. [2006 OL]
The bond in a molecule of hydrogen gas is formed by a shared pair of electrons.
Name the type of bond found in hydrogen gas.
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9. [2008]
Some atoms join together by sharing pairs of electrons. This is called covalent
bonding.
Draw a diagram showing the covalent bonding in a molecule of water.
10. [2009]
The diagram shows a group of water molecules with one enlarged below with its
constituent atoms identified by their atomic symbols. Water molecules are very
tiny, one teaspoon of water contains approximately 2 × 1023 molecules.
(i) Name the type of bonding in the water molecule.
(ii) Describe this type of bond.
(iii) Name one other compound with this type of bonding.
Other Test Questions
1. Name one property of ionic and one property of covalent compounds:
2. Draw a diagram (using suitable circuit symbols) of the circuit needed to investigate the ability of ionic and
covalent compounds to conduct electricity.
3. How could you tell whether a compound was ionic or covalent?
4. Complete the following table detailing the results of the investigation:
Compound Tested
Ionic
Table salt solution
Oil
Dilute acid
Covalent
5. Draw a diagram showing the ionic bonding in the compound magnesium oxide.
6. Name the type of bonding present in a crystal of sodium chloride.
Give one property of this type of bonding:
7. In a negatively charged ion the number of
is greater than the number of
.
8. In a positively charged ion the number of
is greater than the number of
.
9. Sodium Chloride is an ionic substance, what is an ionic bond?
10. State one property of an ionic substance.
11. Draw a diagram showing the bonding in a water molecule.
12. Underline the two elements in the list below that normally exist as molecules:
Hydrogen
Chlorine
Helium
Aluminium
Gold.
13. List two properties of compounds which are formed by sharing electrons.
14. Draw a labelled diagram to show the type of bonding in an oxygen molecule
15. The oxygen molecule can only achieve a stable arrangement of a full outer shell by sharing electrons.
(i) Name the type of bond formed by the oxygen molecule.
(ii) List two properties of compounds which are formed by sharing electrons.
15
Chemistry: 8. The alkali and alkaline earth metals
Syllabus
OC48 Describe the general properties of the alkali metals and understand that alkali metals are in Group I of the
Periodic Table and have similar properties
OC49 Describe the reactions of the alkali metals with air and water (word equations for reaction with water)
OC50 Understand that Group II elements are the alkaline earth metals
Student Notes
The periodic table
Elements in the periodic table are arranged in order of increasing atomic number.
However within this they are also arranged according to another pattern.
Each column is known as a group and has similar properties
In this chapter we concentrate on just the first two columns (known as Group 1 and Group 2)
Group One - The alkali metals
All the elements in Group 1 are known as the alkali metals.
The first three elements in this group are Lithium (Li), Sodium (Na) and Potassium (K)
All the elements in Group 1 share similar properties
1.
2.
3.
4.
5.
Properties of the alkali metals
They have one electron in their outer shell.
This means that they are all very reactive (and as a result are stored in oil).
They are soft and easy to cut.
They are shiny when cut but quickly lose their shine when exposed to air.
Reactivity increases as you go down, so lithium is the least reactive and potassium is the most reactive.
Group 8/0
All the elements in Group 8 (also known as Group 0) have a full outer shell and so all these elements are very
un-reactive.
16
Reactions of the alkali metals with air
Alkali metals react with air and quickly lose their shine to form a metal oxide.
metal
+
oxygen
lithium
lithium + oxygen → lithium oxide
4Li
+ O2
→
2Li2O
→
metal oxide
sodium
sodium + oxygen → sodium oxide
4Na
+ O2
→
2Na2O
Reactions of the alkali metals with water
Alkali metals react vigorously with water.
When dropped in water they float and fizz around and may even catch fire
metal
lithium + water
+
water
→
lithium
→ lithium hydroxide + hydrogen
metal hydroxide
sodium + water
+
hydrogen
sodium
→ sodium hydroxide + hydrogen
Group Two - The alkaline earth metals
Group II elements are known as the alkaline earth metals
All the elements in Group 1 are known as the alkaline earth metals.
The first three elements in this group are beryllium (Be), magnesium (Mg) and calcium (Ca).
17
Exam Questions
1. [2007 OL]
Complete the following sentence below inserting the correct words from the list on the right.
All the known _________________ are listed in the ______________________________.
2. [2007]
Give two properties of alkali metals.
Atoms
Elements
Compounds
Periodic table
3. [2008]
The diagram shows the first twenty elements in their positions in the periodic table. The number given with each
element is the atomic number of that element.
(i) By what name are group two metals known?
(ii) Why are the noble gases, group 8/0, very chemically unreactive?
4. [2006]
(i) Show, clearly using shading and labelling, the location of the alkaline earth metals on the blank periodic table
given.
(ii) Name an alkaline earth metal.
18