Topic 8- Chemical Analysis

Question 1

What is a pure substance?

Answer 1

Usually when you refer to a SUBSTANCE as being PURE you mean that NOTHING has been ADDED to it, so it’s UNADULTERATED and in its NATURAL STATE.

E.g. pure orange juice is taken from oranges with no colouring or sweetness added. Also pure milk.

Question 2

Explain what a pure substance is in chemistry.

Answer 2

In CHEMISTRY, a pure substance is something that only contains ONE COMPOUND or ELEMENT throughout- not mixed with anything else.

So orange juice is considered to be a mixture in chemistry, because it contains a mix of water, vitamin C, fruit sugars, minerals etc.

Question 3

Define what a pure substance is in chemistry.

Answer 3

A substance that only contains ONE compound or element throughout.

Question 4

What is the melting and boiling points of pure substances?

Answer 4

A chemically pure will MELT or BOIL at a SPECIFIC temperature.

Question 5

How can you test a substance for purity (to see if its pure)?

Answer 5

You can test the purity of a sample by measuring its MELTING or BOILING POINT and comparing it with the melting or boiling point of the PURE SUBSTANCE (which you can find from a DATA BOOK).

Question 6

How can you estimate the purity of a substance once you’ve got the results of its melting and boiling points?

Answer 6

The CLOSER your measured value is to the actual melting or boiling point, the PURER your sample is.

Question 7

What will impurities in a sample do to a substance’s melting and boiling points?

Answer 7

Impurities in your sample will:

• LOWER the MELTING POINT and INCREASE the MELTING RANGE of your substance.
• INCREASE the BOILING POINT and may result in your sample boiling over a wider RANGE of temperatures.

Question 8

What is a formulation?

Answer 8

A formulation is a MIXTURE that has been designed as a USEFUL product

Question 9

What is the structure of useful products?

Answer 9

Many products are complex mixtures in which each chemical has a particular purpose.

Question 10

How are formulations made?

Answer 10

A formulation is made by following a formula (recipe).

It is made by mixing together many different substances/components.

Each component in a formulation is present in a MEASURED QUANTITY, and CONTRIBUTES to the properties of the formulation so that it meets its REQUIRED FUNCTION.

Question 11

Give an example of a formulation…

Answer 11

For example, paints are formulations composed of:

1) PIGMENT- gives the paint colour
2) SOLVENT- used to dissolve the other components and altar the viscosity.
3) BINDER (resin) - forms a film that holds the pigment in place after it’s been painted on.
4) ADDITIVES - added to further change the physical and chemical properties of the paint.

Depending on the PURPOSE of the pain, the CHEMICALS used and their AMOUNTS will be changed so the paint produced is right for the job.

Question 12

Are all mixtures formulations?

Answer 12

A formulation is a mixture but NOT all mixtures are formulations. To be a formulation, a mixture has to be a useful product, have been made with a precise purpose in mind and have its components present in particular, carefully measured quantities.

Question 13

Why are formulations really important in the pharmaceutical industry?

Answer 13

Formulations are really important in the pharmaceutical industry. For example, by altering the formulation of a pill, chemists can make sure it delivers the drug to the correct PART OF THE BODY at the right CONCENTRATION, that it’s CONSUMABLE and has a long enough SHELF LIFE.

Question 14

Give some examples of formulations found in everyday life…

Answer 14

Formulations include:

• fuels
• cleaning agents
• paints, medicines
• alloys
• fertilisers
• foods

Question 15

What is chromatography?

Answer 15

CHROMOTOGRAPHY is an analytical method used to SEPARATE the substances in a mixture. You can then use it to IDENTIFY/analyse the substances.

There are various types of chromatography but we learn about paper chromatography.

Question 16

What are the phases of chromatography?

Answer 16

This technique relies on a substance’s relative attraction to the paper.

1) A MOBILE PHASE- where the molecules CAN move. This is always a LIQUID or a GAS.
2) A STATIONARY PHASE- where the molecules CAN’T move. This can be a SOLID or a really THICK LIQUID.

Question 17

Explain how paper chromatography separates mixtures.

Answer 17

During a chromatography experiment, the substances in the sample constantly MOVE between the mobile and the stationary phases- and equilibrium is formed between the two phases.

The mobile phase moves through the stationary phase, and anything DISSOLVED in the mobile phase moves with it.

Question 18

What does the rate of a chemicals movement in chromatography depend on?

Answer 18

*Separation depends on the distribution of substances between the phases.

How quickly a chemical MOVES depends on how it’s DISTRIBUTED between the two phases- whether it spends more time in the mobile phase or the stationary phase.

Question 19

Explain how substances are separated from a mixture during chromatography?

Answer 19

Substances that have a stronger attraction to the paper tend to move more slowly than substances having a stronger attraction for the solvent.

In this way, mixtures of substances can be separated.

Question 20

What is the method for chromatography?

Answer 20

• Draw a pencil line on the paper near the bottom, using a ruler. Pencil is used as ink would run.
• Spot samples carefully onto the pencil line.
• Hang in a baker of solvent, ensuring that the pencil line is above the solvent.
• Close the lid and allow the solvent to rise up the paper.
• When near the top, remove the paper and quickly mark with pencil where the solvent reached. This is called the solvent front.
• Allow the paper to dry.

Question 21

What does each spot on a chromatogram represent?

Answer 21

Each spot represents a different substance.

Question 22

What is a Rf value?

Answer 22

An Rf value (retension factor) is the RATIO between the distance travelled by the DISSOLVED SUBSTANCE (the solute) and the distance travelled by the SOLVENT.

The FURTHER through the stationary phase a substance moves (further the ink moves), the LARGER the Rf value.

Question 23

How can you calculate the Rf value?

Answer 23

The ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent can be expressed as its Rf value:

Rf = distance travelled by substance ÷ distance travelled by solvent


Question 24

What is the Rf value of compounds?

Answer 24

Different compounds have different Rf values in different solvents, which can be used to help identify the compounds.

Question 25

Why is a chromatography experiment carried out?

Answer 25

Chromatography is carried out to see if a certain substance is present in a mixture.

Question 26

How can find out if a certain substance is present in a mixture?

Answer 26

To do this, you run a PURE SAMPLE of that substance (a reference) alongside the unknown mixture.

If the Rf values of the reference and one of the spots in the mixture MATCH, the substance may be present (although you haven’t yet proved they’re the same).

Question 27

Does the solvent change the number of spots produced in a compound and a pure substance?

Answer 27

The compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents.

Question 28

What is the Rf value dependent on?

Answer 28

The Rf value is DEPENDENT on the solvent- if you CHANGE THE SOLVENT the Rf value for a substance will CHANGE.

Question 29

How can you prove that a substance is present in a mixture?

Answer 29

You can test test both the MIXTURE and the REFERENCE in a number of the spots in the mixture in all solvents, then it’s likely the reference compound is PRESENT in the mixture.

Question 30

How can you prove that a substance is not present in a mixture?

Answer 30

If the spots in the mixture and the spot in the reference only have the same Rf value in SOME of the solvents, then the reference compound ISN’T present in the mixture.

Question 31

How do you find the distance travelled by a substance?

Answer 31

To find the distance travelled by a substance you need to measure from the BASELINE (which should be the centre of the spot of the original mixture) to the CENTRE of the spot of the substance that’s separated out.

Question 32

How can you tell if your Rf value is incorrect?

Answer 32

Rf values are always less than 1. So if you get an answer that’s greater than 1 you need to go back and do the calculation again- you’ll have made a mistake somewhere.

Question 33

In chromatography, what’s the name of the phase which can’t move?

Answer 33

The stationary phase.

Question 34

What conditions affect how long molecules are in the mobile phase?

Answer 34

How soluble they are in the solvent and how attracted they are to the paper.

Question 35

Why is it useful if you can identify a gas released from a chemical reaction?

Answer 35

Many reactions you come across release a gas as a product. Identifying these gases can be a useful way of telling if a particular reaction occurred.

Question 36

How can you test for chlorine?

Answer 36

The test for chlorine uses litmus paper.

When damp litmus paper is put into contact with chlorine gas, the paper becomes bleached and turns white.
(It may turn red for a moment first though-that’s because a solution of chlorine is acidic).

Question 37

How can you test for oxygen?

Answer 37

The test for oxygen uses a glowing splint inserted into a test tube of the gas. The splint relights in oxygen.

Question 38

How can you test for carbon dioxide?

Answer 38

The test for carbon dioxide uses an aqueous solution of calcium hydroxide (lime water). When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky (cloudy).

Question 39

How can you test for hydrogen?

Tip- Make sure to wear appropriate safety protection and carry out a risk assessment before doing any of these tests.

Answer 39

The test for hydrogen uses a burning splint held at the open end of a test tube of the gas.

Hydrogen burns rapidly with oxygen in the air to form H2O causing a SQUEKY POP sound.

Question 40

What are ionic compounds?

Answer 40

Ionic compounds are made up of positive ions (cations) and negative ions (anions).

Question 41

How can you find which ions a compound contains?

Answer 41

To do this there are different tests that you can carry out which identify whether specific ions are present.

Question 42

What is a flame test?

Answer 42

A flame test is a technique used to identify metal ions.

Question 43

How can flame tests be used to identify metal ions?

Answer 43

Compounds of some metals burn with a CHARACTERISTIC COLOUR.

So you can test for various metal ions by heating your substance and seeing whether it BURNS with a distinctive flame.

Question 44

Explain how flame tests are carried out.

Answer 44

1) CLEAN a PLATINUM wire loop by dipping it in some DILUTE HCl and then hold it in the BLUE flame from a bunsen burner UNTIL it burns without ANY colour.
2) Dip the wire loop into the metal compound you want to test.
3) Put it into the blue bunsen flame and observe the colour. Record the colour of the flame.

Question 45

How can you identify different ions from flame tests?

Answer 45

You can use these COLOURS to DETECT and IDENTIFY different ions.

Question 46

What are the colours of the flames produced by metal ions?

Answer 46

Lithium ions, Li+, burn with a crimson flame.

Sodium ions, Na+, burn with a yellow flame.

Potassium ions, K+, burn with a lilac flame.

Calcium ions, Ca 2+, burn with an orange-red flame.

Copper ions, Cu 2+, burn with a green flame.

Question 47

What is the one drawback of using the flame test to identify metal ions?

Answer 47

It only works for samples that contain a single metal ion.

If the sample tested contains a MIXTURE of metal ions, the flame colours of some IONS may be MASKED (blocked out) by the colours of others.

Therefore, some colours may not be noted.