​Intro image

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​To test for carbon dioxide. the test is to force the sample that you believe may be carbon dioxide in to a solution of lime water. if the sample does contain carbon dioxide then the solution will turn cloudy, if it does not then the sample cannot contain carbon dioxide.

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Oxygen is a  very reactive gas. It makes up approximately 20%  of our atmosphere.  

​The standard test for oxygen is to place a glowing splint into a test tube that may contain the gas.

If the splint glows brighter and/or relights into flame then it is a positive result for oxygen.

​Hydrogen is a flammable gas. It burns in air to produce water.

A convenient test for hydrogen is to put a lighted splint in the mouth of a test tube full of the gas . The gas will burn with a characteristic squeaky pop if hydrogen is present.

Hydrogen gas can be used as a fuel. 

When hydrogen burns in air it combines with oxygen to form water.

This reaction releases energy and it therefore an example of an exothermic reaction 

Some squeaky pops 

​Task 1.

Watch the video to see and hear the students using hydrogen to make some squeaky pops.

Are all squeaky pops the same?

Try to explain your answer.

Task 2.

Find out and write about three ways to make hydrogen gas. In each case say what the reactants are, what the products are and   write down chemical equations for the reactions.

Task 3.

Some say that hydrogen is a perfect fuel since it is not polluting.

Explain why hydrogen is not pollution when it burns.

Others say that hydrogen is not really a clean fuel. Can you explain why they can make this claim? 

The periodic table lists the elements in order of increasing atomic number

​The modern periodic table provides a list of all the known elements categorized in increasing order of increasing atomic number

Each row across the periodic table is known as a period.

Each column is known as a group.

The period number tells us the number of occupied electron shells ion a given atom. The group number shows the number of electrons in the outermost shell of the atom.

The electrons in an atom arrange themselves in shells ( or levels) around the nucleus. The electrons fill the innermost shells first . Each shell can contain a maximum number of electrons. For GCSE and IGCSE we use a simplified model of electron shells. 

This model works well for explaining the properties of the first 20 elements in the periodic table  - a more complex model is needed to explain the structure of atoms above atomic number 20.

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Electron shells have a maximum number of electrons.

Working out  electron configurations 

A carbon atom has 6 protons and therefore 6 electronsThe electrons are arranged in two shells; 2 electrons in the first shell and 4 electrons in the second shell. The electron configuration of a carbon atom can be therefore represented as: 2, 4 

Carbon atoms have an electron configuration of 2,4

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Ruby is a crystalline form of Aluminium Oxide

A crystal or crystalline solid is a solid material whose constituent particles (such as atoms, molecules, or ions) are arranged in a highly ordered structure

How do you know when a substance is crystalline?
Metal Crystals

Bismuth Crystals.

Metals will form  crystalline structures because their atoms can bond together in a highly organised fashion

​Crystals can form when a molten solid cools and solidifies. They can also form when a saturated solution is cooled. This happens because the solubility of most solid solutes  decreases as the temperature is lowered. 

The solute can no longer stay in solution and therefore forms a solid precipitate. The precipitate can be crystalline.

Crystallisation can therefore be used to separate a solid solute from its solvent.

Pure crystal 

Task 1
Watch  the video and answer the following:
  1. When recrystallising in order to purify a solid, how much solvent is used
  2. There is a mistake in the apparatus set up at the beginning of the video. Describe the mistake and explain why it would be dangerous to heat the apparatus as shown.
  3. Explain why the filter funnel is heated by a hot water jacket?
  4. Why do crystals then form in the filtrate?
  5. How are these crystals then separated?

The crystals obtained this way can then be dissolved and recrystallised to improve the purity further.​

Crystallisation close up 

Task 2

This video helps you to visualise how molecules ( or atoms) can stick together to form an orderly arrangement and therefore form crystals.

  1. What shape is used in the video to represent the sugar molecules ?
  2. The four images below show four different ways of "modelling" the arrangement of atoms or molecules forming a crystalline structure. Consider each model in turn. For each model try to say what is good about it and what might be "inaccurate" about it. This could be presented as a table of strengths and weaknesses.
  3. It is often said that crystallising something slowly will produce lager and more regular crystals. Can you explain this?
Spheres can be used to model the way atoms and molecules might stack together.

In this video you see how scientist use bubbles in a soap solution to model the behaviour of particles in a crystal. 

Fractional distillation in the laboratory

A fractionating column can be added to a simple distillation apparatus. The column is used to achieve a good separation of one liquid from another. 

The column is often packed with glass beads which cause repeated condensation and evaporation. This can give a much purer condensate ( product) than when using simple distillation.

By controlling the temperature carefully the different fractions ( components) in the mixture of liquids can be separated from each other.

The most volatile components of the mixture will be the first to be extracted.

Industrial fractional distillation

Fractional distillation is used to separate crude oil into its different fractions. The most volatile fractions rise to the top of the column.

Task 1. Simple or fractional?

Use the information on this post and the one called "simple distillation" to explain the differences between Simple and Fractional distillation.

Task 2. From thin air

Watch the first  2 minutes 25 seconds of this video.

Answer  the following questions:

  • ​what are the main components of air?
  • what is the percentage of each component - illustrate this with an appropriate chart
  • what are the main processes required for the separation of the components in air?
  • explain what liquefaction is. How is it achieved?
  • how is liquid air separated into its different fractions?
  • explain why this process works . Use the following words: volatile, boiling point , separate, evaporate. 
Water cycle - Distillation on a global scale

Distillation: the action of purifying a liquid by a process of heating and cooling

The water cycle.. 

Distillation on a very large scale...

You will be very familiar with the ideas illustrated in this graphic explaining the water cycle. The continual evaporation of water ( the solvent) from the sea ( the solution) - followed by cooling and condensation of the water vapour - means that the water that falls on the land is "distilled" and free from the salt 

Task 1

Explain in no more than 50 words why the water cycle makes the sea salty.

words to use:

dissolve, soluble, volatile, evaporate, condense

Task 2:

Find out about the salt content of the Dead Sea. Write down your findings and explain them.

Simple distillation in the laboratory

This video shows how the water cycle can be simulated in the laboratory.  In this example you can see pure water being distilled from salty water. Watch the video closely.

Task 3: 

  • The word "retort" is used several times. Explain in your own words what a retort is .
  • Explain why the water condenser has cold water fed in at the bottom and not the top.
  • Distillation is one way of making drinking water from seawater. This is often an unfeasibly expensive way of producing drinking water. Explain why.
Simple laboratory distillation

That's the spirit.. 

Alcoholic spirits are prepared by distillation

Fermentation can be used to produce a dilute solution such as beer or wine. These solutions can be further distilled to produce a spirit which contains much more alcohol and is consequently much more intoxicating.

A vintage Industrial scale distillation.

​Task 4.

Study this engraving of the distillation of ethanol. Make a copy of the drawing and label it with the same labels as the other simple distillation diagram.

Learn the terms: 

pure water can be extracted from water based inks by using distillation

Task 5. 

Study the video carefully. Two students are using distillation to extract pure water from some water soluble ink.

The students have made at least three deliberate mistakes.

Describe the mistakes and explain what they should have done instead in order to complete the task safely and correctly.

A colourful Chromatogram.

Chromatography is another important separation technique.  Several different types of chromatography exist. 

All chromatography relies upon the idea that different solutes have different solubilities. This fact is used in chromatography to separate and identify the separate components in a mixture. 

Here we will consider the simplest Chromatogaphic method - paper chromatography.

Chromatography gets its name ( Chroma="colour" in Greek) from the fact that mixtures which initially look like one uniform colour can be separated to produce many different colours.
Not all chromatograms are coloured.

In the Lab

All chromatography involves a mobile phase and a stationary phase.  The mobile phase moves through the static phase. The solutes which are more soluble (in the mobile phase) will travel faster through it than other less soluble solutes.

Task 1. Watch this video carefully and write answers to the questions:

  1. In paper chromatography what is the stationary phase? 
  2. In the video what is the name of the mobile phase
  3. Name the four substances which are separated from the spinach leaf extract. List them in order of their solubility in the mobile phase. ( put the most soluble first)
Butterfly or ink blobs?

​A slightly different chromatographic method.

A paper filter

Filter first  

Filtration is any of various mechanical, physical or biological operations that separate solids from fluids (liquids or gases) by adding a medium ( a filter) through which only the fluid can pass. The fluid that passes through is called the filtrate.

Filtration is a simple technique used  to separate insoluble solids from a solutionSometimes it is the filtrate that we want to isolate ( as in the making of a cup of coffee), on other occasions it is the residue that is left behind in the filter  that is of value.

In the laboratory 

suction filtration

​In the laboratory suction filtration can speed up the process and help to dry the insoluble residue on the filter paper.

Task 1

Make a copy of the suction filtration diagram. Add labels to the diagram to help explain why a vacuum pump is used.

What's the connection?

figure 1
figure 2
figure 3
figure 4

Task 2. Compare the four images above. What is the connection between all four? Which one is the odd one out  and why ?

(Hint: There is more than one acceptable answer - so long as you can justify it..)

Three of the Noble gases

Elements - close up

Here we look at the elements and some compounds - close up. 

Some elements (non-metal) have atoms which join up with one another to form small molecules. Some other non- metal elements have atoms which join up to form giant covalent lattices.

A monatomic gas

The particles which make up the the Noble Gases are single atoms; they do not join up with one another. They are monatomic. 

These atoms are very unreactive and do not easily form compounds

Simple molecular elements. They are often gases at room temperature but can be liquid or easily melted solids.

A solid element with small molecules
A gaseous element with small molecules

 Giant lattices. These elements form giant lattices have high melting points and form crystalline structures

A giant covalent lattice
A metallic element

 Compounds - close up

A compound containing small molecules
A compound which forms a giant covalent lattice
A compound which is made of a giant ionic lattice
A compound made of long chain molecules