C12 - Chemical Analysis

Question 1

What is a pure substance

Answer 1

one that is made up of just one substance. That substance can be either an element or a compound

Question 2

what is an impure substance

Answer 2

a mixture of two or more different elements or compounds

Question 3

What is different between the boiling and melting points of pure and impure substances

Answer 3

Pure substances melt and boil at specific temperatures (fixed points) and impure substances don’t have a sharp melting or boiling point and change state over a range of temperatures

Question 4

What are formulations

Answer 4

Useful mixtures made up in definite proportions, designed to give a product the best properties possible to carry out its function and

Question 5

What is a mobile and stationary phase

Answer 5

The mobile phase moves through the stationary phase, carrying the components of the mixture being investigated with it

Question 6

What happens if a substance has stronger forces of attraction between itself and the mobile phase than for the stationary phase

Answer 6

The substance will be carried a greater distance in a given time. The further through the stationary and user a substance moves, the larger the Rf value

Question 7

What is the mobile and stationary phase in paper chromatography

Answer 7

The mobile phase is the solvent chosen and the stationary phase is the paper

Question 8

Instructions on how to carry out paper chromatography

Answer 8

1. Draw a base line in pencil (so it doesn’t dissolve in the solvent) near the bottom of the paper and draw crosses at equal distances
2. Place drops of different dyes at equal distances on the base line using a capillary tube
3. Place paper in a beaker with the solvent with the solvent below the base line
4. Then dry the paper and calculate the distance moved by the solvent and the Rf value of each substance

Question 9

What is the Rf value and how is it calculated

Answer 9

The Rf (Retention Factor) value is the ratio between the distance travelled by the dissolved substance (solute) and the distance travelled by the solvent

Rf = distance travelled by substance/ distance travelled by solvent

Question 10

How to tell if a certain substance is present in the mixture

Answer 10

Run a pure sample of that substance (a reference) alongside the unknown mixture and repeat with different solvents. If the Rf values match in all substances, the substance is present

Question 11

How to test for chlorine

Answer 11

Chlorine bleached damp blue litmus was paper and turns it white

Question 12

How to test for Oxygen

Answer 12

if you put a glowing splint inside a test tube containing oxygen, the oxygen will relight the glowing splint

Question 13

How to test for carbon dioxide

Answer 13

Bubble the gas through an aqueous solution of calcium hydroxide (limewater) and it will turn cloudy if the gas is carbon dioxide

Question 14

How to test for hydrogen

Answer 14

If you hold a burning splint at the open end of a test tube containing hydrogen, you’ll get a ‘squeaky pop’. This noise comes from the hydrogen burning quickly in the oxygen in the air to form water

Question 15

How to test for CO3^2- (carbonate) ions in carbonates

Answer 15

1. Add a few drops of a dilute acid to a test tube containing the mystery substance
2. Then connect the test tube to a test tube contains limewater
3. If carbonate ions are present, the solution will effervesce and the reaction will release carbon dioxide which will turn the limewater cloudy

Question 16

How to test for sulfate ions (SO4^2-)

Answer 16

1. Use a dropping pipette to add a couple of drops of dilute hydrochloric acid followed by a couple of drops of barium chloride solution (BaCl2)
2. If sulfate ions are present, a white precipitate of barium sulfate will form

Ba^2+ + SO4^2- —> BaSO4

Question 17

How to test for halide ions and determine which halide is present

Answer 17

Add a couple of drops of dilute nitric acid followed by a couple of drops in f silver nitrate solution to your mystery solution
•A chloride will give a white precipitate of silver chloride Ag+ + Cl- —> -AgCl
•A bromide will give a cream precipitate of silver bromide Ag+ + Br- —> AgBr
•An iodide will give a yellow precipitate of silver iodide Ag+ + I- —> AgI

Question 18

How to test for metal ions using the flame test and what ions it tests for

Answer 18

1. First dip a nichrome wire loop in concentrated hydrochloric acid and heat it to clean it until the flame remains blue
2. Then dip the wire loop into the acid again before dipping it into the metal compound being tested
3. Hold the loop in the blue flame of the bunsen burner and record the colour of the flame

The flame test tests for Lithium, Sodium, Potassium, Calcium and Copper ions

Question 19

How to determine which metal ion is present from the colour of the flame

Answer 19

• Lithium Ions (Li+) produce a Crimson flame
• Sodium ions (Na+) produce a Yellow flame
• Potassium ions (K+) produce a Lilac flame
• Calcium ions (Ca^2+) produce a Orange-Red flame
• Copper ions (Cu^2+) produce a Green flame

Question 20

How to test for metal ions with sodium hydroxide and what ions it tests for

Answer 20

Many metal hydroxides are insoluble and precipitate our of solutions when formed and some of these hydroxides have a characteristic colour
In this test, add a few drops of sodium hydroxide solution to a solution of your mystery compound

This tests for Calcium, Copper (II), Iron (II), Iron (III), Aluminium and Magnesium ions

Question 21

How to identify which metal ion is present from adding sodium hydroxide

Answer 21

• Calcium (Ca^2+) - white precipitate
• Copper (II) - Blue precipitate
• Iron (II) - Green precipitate
• Iron (III) - Brown precipitate
• Aluminium (Al^3+) - white precipitate at first but redissolves in excess NaOH to become colourless
• Magnesium (Mg^2+) - white precipitate

Question 22

Why is light released during flame emission spectroscopy?

Answer 22

A sample is placed in a flame and as the ions heat up, their electrons become excited (they move up to a higher energy level). When the electrons drop back to their original energy levels, they release energy as light.

Question 23

How does flame emission spectroscopy work

Answer 23

A sample is placed in a flame and releases energy as light. The light passes through a spectroscope which can detect different wavelengths of light to produce a line spectrum

Question 24

How to determine the substance and concentration from a line spectrum

Answer 24

• The combination of wavelengths emitted by an ion depend on its charge and electron arrangement. This means that no two ions will produce the same wavelengths and so will have different line different line spectrums
• The intensity of the spectrum indicates the concentration of the ion in the solution

Question 25

What are the pros of instrumental methods

Answer 25

• Small sampler sizes can be analysed as the methods are very sensitive
• Very fast and tests can be automated
• Very accurate

Question 26

Cons of Instrumental methods

Answer 26

• Require training to use
• Results often need to be compared to known substances
• Expensive