Substances can be analysed using a variety of methods including paper chromatography, gas chromatography and mass spectrometry.
Methods of analysis
Chemists can use many different methods to analyse substances. Some methods rely on chemical analysis, while others rely on machines.
Paper chromatography
Paper chromatography is used to analyse coloured substances, such as the coloured pigments in plants and artificial colours used as food additives.
Paper chromatography works because some of the coloured substances are better at dissolving in the liquid than they are at bonding with the paper, so they travel further up the paper.
The animation shows what happens during paper chromatography.
Two substances are likely to be the same if they have the same colour and they travel the same distance up the paper.
Instrumental methods of analysis
Instrumental methods of analysis rely on machines. There are several different types of instrumental analysis. Some are suitable for detecting and identifyingelements, while others are better suited to compounds. In general, instrumental methods of analysis are:
Fast
Accurate (they reliably identify elements and compounds)
Sensitive (they can detect very small amounts of a substance in a small amount of sample)
The relative atomic mass of an element (Ar) is an average value for the isotopesof the element. For example, the Ar for chlorine is 35.5 because it contains two different isotopes.
Chlorine
Chlorine has two isotopes:
Isotope
Protons
Electrons
Neutrons
17
17
35 - 17 = 18
17
17
37 - 17 = 20
75 per cent of chlorine atoms are 35Cl and 25 per cent of chlorine atoms are 37Cl.
This means that in 100 chlorine atoms, 75 will be 35Cl and 25 will be 37Cl.
The total Ar for these chlorine atoms will be (75 × 35) + (25 × 37) = 2625 + 925 = 3550.
So the average Ar for chlorine is 3550 ÷ 100 = 35.5.
A standard atom
The mass of the 12C isotope is the 'standard atom' that the masses of other atoms are compared to. The Ar of 12C is defined as 12. This means, for example, that one 24Mg atom has twice the mass of a 12C atom.
For your exam, you will need to know what relative formula mass is. You should also be able to work out the relative formula mass of a substance when given its formula.
The symbol for relative formula mass is Mr. The symbol for relative atomic mass is Ar. You will be given any Ar values you need in the examination. The table shows some of these values:
Element
Relative atomic mass (Ar)
H
1
C
12
O
16
Na
23
Mg
24
Working out Mr
To find the relative formula mass of a substance, you just add together the relative atomic mass values for all the atoms in its formula. Here are three examples:
Example 1
Find the Mr of carbon monoxide, CO
Mr = 12 + 16 = 28
Example 2
Find the Mr of sodium oxide, Na2O
Mr = (23 × 2) + 16 = 46 + 16 = 62
Example 3
Find the Mr of magnesium hydroxide, Mg(OH)2
Mr = 24 + 2 × (16+1) = 24 + 34 = 58
(Remember that there are two of each atom inside the brackets)
Moles
The relative formula mass of a substance - shown in grams - is called one moleof that substance. For example, the Mr of carbon monoxide (CO) is 28. This means that one mole of carbon monoxide has a mass of 28 g. You should be able to see that:
14 g of carbon monoxide contains 14 ÷ 28 = 0.5 moles
56 g of carbon monoxide contains 56 ÷ 28 = 2 moles
The atoms of a particular element will all have the same number of protons. Their atomic number will be the same. However, the atoms of an element can have different numbers of neutrons - so their mass numbers will be different.
Atoms of the same element with different numbers of neutrons are calledisotopes. The different isotopes of an element have identical chemical properties. However, some isotopes are radioactive.
Chemical symbols
The full chemical symbol for an element shows its mass number at the top, and atomic number at the bottom. Here is the full symbol for carbon:
It tells us that a carbon atom has six protons. It will also have six electrons, because the number of protons and electrons in an atom is the same.
The symbol also tells us that the total number of protons and neutrons in a carbon atom is 12. Note that you can work out the number of neutrons from the mass number and atomic number. In this example, it is 12 – 6 = 6 neutrons.
Isotopes of hydrogen
Most hydrogen atoms consist of just one proton and one electron, but some also have one or two neutrons. The table summarises these isotopes.
The mass number of an atom is its total number of protons and neutrons. Isotopes are atoms of an element with different numbers of neutrons. The relative formula mass of a compound is found by adding together the relative atomic masses of all the atoms in the formula of the compound. The relative formula mass of a substance in grams is one mole of that substance.
Subatomic particles
Structure of the atom
Each atom consists of a nucleus containingprotons and neutrons, with electronsarranged around it in energy levels.
Relative masses of subatomic particles
Name of particle
Relative mass
Proton
1
Neutron
1
Electron
Very small (1/1836)
Mass number and atomic number
The mass number of an atom is never smaller than the atomic number. It can be the same, but is usually bigger:
The mass number of an atom is the total number of protons and neutrons it contains
The atomic number of an atom is the number of protons it contains
Notice that most of the mass of an atom is found in the nucleus.
Graphite is soft and slippery because there are only weak intermolecular forces between its layers.
From left to right - graphite, diamond, silica
Graphite is a good conductor of heat and electricity. This is because, like metals, graphite contains delocalised electrons. These electrons are free to move through the structure of the graphite.
Fullerenes
Structure of a buckminsterfullerene molecule - a large ball of 60 atoms
Carbon exists as graphite and diamond, but it can also form fullerenes. These are cages and tubes with different number of carbon atoms. Buckminsterfullerene is one type of fullerene. Its molecules are spherical and contain 60 carbon atoms.
Fullerenes may be used for drug delivery systems in the body, in lubricants and as catalysts.
The tube fullerenes are called nanotubes. These are very strong. They are useful in reinforcing structures where lightness and strength are needed - for example, in tennis racket frames.
Working with nanoparticles is called nanotechnology
A nanometre, 1 nm, is one billionth of a metre (or a millionth of a millimetre). Nanoparticles range in size from about 100 nm down to about 1 nm. They are typically the size of small molecules, and far too small to see with a microscope.
Properties and uses of nanoparticles
Nanoparticles have a very large surface area compared with their volume, so they are often able to react very quickly. This makes them useful as catalysts to speed up reactions. They can, for example, be used in self-cleaning ovens and windows.
Nanoparticles also have different properties to the same substance in normal-sized pieces. For example, titanium dioxide is a white solid used in house paint and certain sweet-coated chocolates. Titanium dioxide nanoparticles are so small that they do not reflect visible light, so cannot be seen. They are used in sun screens to block harmful ultraviolet light without appearing white on the skin.
In addition to new cosmetics such as sun screens and deodorants, nanoscience may lead to the development of:
New catalysts
New coatings
New computers
Stronger and lighter building materials
Sensors that detect individual substances in tiny amounts
An alloy is a mixture of two or more elements, where at least one element is a metal. Many alloys are mixtures of two or more metals.
Layers
Alloys contain atoms of different sizes. These different sizes distort the regular arrangements of atoms. This makes it more difficult for the layers to slide over each other, so alloys are harder than the pure metal.
It is more difficult for layers of atoms to slide over each other in alloys
Copper, gold and aluminium are too soft for many uses. They are mixed with other metals to make them harder for everyday use. For example:
Brass - used in electrical fittings - is 70 per cent copper and 30 per cent zinc
18-carat gold - used in jewellery - is 75 per cent gold and 25 per cent copper and other metals
Duralumin - used in aircraft manufacture - is 96 per cent aluminium and 4 per cent copper and other metals
Shape memory alloys
Shape memory alloys can return to their original shape after being bent or twisted. Nitinol is a shape memory alloy made from nickel and titanium. It is used in dental braces and spectacle frames.
Layers of atoms slide over each other when metals are bent or stretched
Metals are malleable - they can be bent and shaped. This is because they consist of layers of atoms. These layers can slide over one another when the metal is bent, hammered or pressed.
Metals - Higher tier
Metals form giant structures in which electrons in the outer shells of the metal atoms are free to move. The metallic bond is the force of attraction between these free electrons and metal ions. Metallic bonds are strong, so metals can maintain a regular structure and usually have high melting and boiling points.
Metals are good conductors of electricity and heat. This is because the free electrons can move throughout the metal.
Ionic bonds form when a metal reacts with a non-metal. Metals form positive ions, while non-metals form negative ions. Ionic bonds are the electrostatic forces of attraction between oppositely charged ions.
Melting points and boiling points
Ionic bonds are very strong so a lot of energy is needed to break them. Ionic compounds contain many of these strong bonds so they have high melting and boiling points.
Conduction of electricity
Ionic compounds conduct electricity when they are dissolved in water or when they are melted. This is because their ions are free to move and carry the current. However, ionic compounds do not conduct electricity when they are solid. This is because their ions cannot move around in their lattice structure.
Polymershave properties which depend on the chemicals they are made from, and the conditions in which they are made. For example, poly(ethene) can be low-density or high-density depending upon thecatalystand reaction condition used to make it. The table summarises some differences in their properties:
LDPE low-density poly(ethene)
HDPE high-density poly(ethene)
Branches on polymer molecules
Many
Few
Relative strength
Weak
Strong
Maximum useable temperature
85°C
120°C
Thermosoftening polymers
Polymer with no cross-links
Thermosoftening polymers soften when heated and can be shaped when hot. The shape will harden when it is cooled, but can be reshaped when heated up again. Poly(ethene) is a thermosoftening polymer. Its tangled polymer chains can uncoil and slide past each other, making it a flexible material.
Thermosetting polymers
Polymer with cross-links
Thermosetting polymers have different properties to thermosoftening polymers. Once moulded, they do not soften when heated and they cannot be reshaped. Vulcanised rubber is a thermoset used to make tyres. Its polymer chains are joined together by cross-links, so they cannot slide past each other easily.
Simple molecular substances consist of molecules in which the atoms are joined by strong covalent bonds. However, the molecules are held together by weak forces so these substances have low melting and boiling points. They do not conduct electricity.
Giant covalent structures contain many atoms joined together by covalent bonds to form a giant lattice. They have high melting and boiling points. Graphite and diamond have different properties because they have different structures. Graphite conducts heat and electricity well because it also has free electrons.
Simple molecules
Covalent bonds form between non-metal atoms. Each bond consists of a shared pair of electrons, and is very strong. Covalently bonded substances fall into two main types:
Simple molecules
Giant covalent structures
Simple molecules
A molecule of carbon dioxide
These contain only a few atoms held together by strong covalent bonds. An example is carbon dioxide (CO2), the molecules of which contain one atom of carbon bonded with two atoms of oxygen.
Properties of simple molecular substances
Low melting and boiling points - This is because the weak intermolecular forces break down easily.
Non-conductive - Substances with a simple molecular structure do notconduct electricity. This is because they do not have any free electrons or an overall electric charge.
Higher tier only
Hydrogen, ammonia, methane and water are also simple molecules with covalent bonds. All have very strong bonds between the atoms, but much weaker forces holding the molecules together. When one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong covalent bonds. Simple molecular substances are gases, liquids or solids with low melting and boiling points.
The animation shows how the weak intermolecular forces between water molecules break down during boiling or melting:
Macromolecules
Macromolecules have giant covalent structures. They contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds. Their atoms are arranged into giant lattices, which are strong structures because of the many bonds involved. Substances with giant covalent structures have very high melting points, because a lot of strong covalent bonds must be broken. Graphite, for example, has a melting point of more than 3,600°C.
Diamond
Diamond
Diamond is a form of carbon in which each carbon atom is joined to four other carbon atoms, forming a giant covalent structure. As a result, diamond is very hard and has a high melting point. It does not conduct electricity.
Graphite
Graphite
Graphite is a form of carbon in which the carbon atoms form layers. Each carbon atom in a layer is joined to only three other carbon atoms.The layers can slide over each other because there are no covalent bonds between them. This makes graphite much softer than diamond. It is used in pencils and as a lubricant. Graphite conducts electricity.
Silica
Silica
Silica, which is found in sand, has a similar structure to diamond. It is also hard and has a high melting point. However, it contains silicon and oxygen atoms instead of carbon atoms.
A covalent bond is a strong bond between two non-metal atoms. It consists of a shared pair of electrons. A covalent bond can be represented by a straight line or dot-and-cross diagram.
Hydrogen and chlorine can each form one covalent bond, oxygen two bonds, nitrogen three, while carbon can form four bonds.
A shared pair of electrons
You will need to understand what covalent bonding is, and to remember some of the properties of molecules that are formed in this way.
A covalent bond forms when two non-metal atoms share a pair of electrons. The electrons involved are in the highest occupied energy levels - or outer shells - of the atoms. An atom that shares one or more of its electrons will complete its highest occupied energy level.
Covalent bonds are strong - a lot of energy is needed to break them. Substances with covalent bonds often form molecules with low melting and boiling points, such as hydrogen and water.
The animation shows a covalent bond being formed between a hydrogen atom and a chlorine atom, to form hydrogen chloride.
After bonding, the chlorine atom is now in contact with eight electrons in its highest energy level - so it is stable. The hydrogen atom is now in contact with two electrons in its highest energy level - so the hydrogen is also stable.
How many bonds?
Atoms may form multiple covalent bonds - that is, share not just one pair of electrons but two or more pairs. Atoms of different elements will form either one, two, three or four covalent bonds with other atoms.
There is a quick way to work out how many covalent bonds an element will form. The number of covalent bonds is equal to eight minus the group number (you can brush up on group numbers by reading through the section in AQA GCSE Science on the Periodic Table). The table below gives more detail on this rule:
Group 4
Group 5
Group 6
Group 7
Example
Carbon
Nitrogen
Oxygen
Chlorine
Number of bonds
8 - 4 = 4
8 - 5 = 3
8 - 6 = 2
8 - 7 = 1
Hydrogen forms one covalent bond. The noble gases in Group 0 do not form any
Representing covalent bonds
Covalent bonds can be represented in several different ways.
Straight lines and models
Straight lines are the most common way to represent covalent bonds, with each line representing a shared pair of electrons. 2D or 3D molecular models are especially useful for showing the relationship between atoms in multiple covalent bonds. Below are some examples of straight lines and images of 3D models.
Models for covalent bonds
Element
Formula
Chemical structure
Ball-and-stick model
Hydrogen
H2
Water
H2O
Ammonia
NH3
Methane
CH4
Double and triple bonds
Note that molecules can have a double covalent bond - meaning they have two shared pairs of electrons - or a triple covalent bond - three shared pairs of electrons. A double covalent bond is shown by a double line, and a triple bond by a triple line.
A molecule of oxygen (O2) consists of two oxygen atoms held together by a double bond, like this:
A molecule of nitrogen (N2) has two nitrogen atoms held together by a triple bond, like this:
Dot-and-cross diagrams - elements
Dot-and-cross diagrams
Dot-and-cross diagrams are used to represent covalent bonds. The shared electron from one atom is shown as a dot, while the shared electron from the other atom is shown as a cross.
When drawing dot-and-cross diagrams for covalent bonds, you only need to show the electrons in the highest occupied energy level, as only these are involved.
The animations show covalent bonds represented by both displayed formulae (which use straight lines to represent bonds) and dot-and-cross diagrams:
Covalent bonding between two hydrogen atoms to form a molecule of hydrogen gas, H2.
Covalent bonding between two oxygen atoms to form a molecule of oxygen gas, O2.
Elements
For your examination, you need to be able to draw dot-and-cross diagrams for hydrogen, chlorine and oxygen.
You do not need to use colours in your answers.
Dot-and-cross diagrams - compounds
You will also need to be able to draw dot-and-cross diagrams representing thecovalent bonds in the molecules of some common compounds:
Hydrogen chloride, HCl
Hydrogen atoms and chlorine atoms can each form one covalent bond. One pair of electrons is shared in a hydrogen chloride molecule (HCl).
Water, H2O
Hydrogen atoms can each form one covalent bond, while oxygen atoms can each form two covalent bonds. Two pairs of electrons are shared in a water molecule (H2O).
Ammonia, NH3
Hydrogen atoms can each form one covalent bond, while nitrogen atoms can each form three covalent bonds. Three pairs of electrons are shared in an ammonia molecule (NH3).
Methane, CH4
Hydrogen atoms can each form one covalent bond, while carbon atoms can each form four covalent bonds. Four pairs of electrons are shared in a methane molecule (CH4).
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