Topic 8 - Chemical analysis

Question 1

What is a pure substance?

Answer 1

A substance which contains a single element or compound and has not been mixed with anything else. They only melt and boil at specific temperatures.

Question 2

How do impurities affect the melting and boiling point of a substance?

Answer 2

Impurities lower the melting point and increase the boiling point

Question 3

What are formulations and how are they made? Give some examples of formulations.

Answer 3

Formulations are mixtures that have been designed for a specific purpose. They are made by mixing components in measured quantities to ensure the product has the required properties.

Examples include:

• – cleaning agents
• – paints
• – food
• – fuels
• – cosmetics
• – fertilisers
• – alloys
• – medicines

Question 4

In paper chromatography, what are the mobile phase and stationary phase?

Answer 4

Mobile phase: solvent and any dissolved inks in it

Stationary phase: chromatography paper

Question 5

What affects the amount of time that molecules spend in each phase?

Answer 5

• – How soluble the molecules are in the solvent

- – How attracted they re to the paper

Question 6

How do you calculate the Rf value?

Answer 6

distance travelled by the substance
Rf value = ——————————————————–
distance travelled by the solvent

Question 7

What are the four tests to identify gases?

Answer 7

Test for hydrogen:
— Hold a lit splint at the end of the test tube, if you hear a squeaky pop sound then hydrogen is present.

Test for oxygen:
— Insert a glowing splint into a test tube, if the splint relights then oxygen is present.

Test for chlorine:
— If damp litmus paper is inserted into chlorine gas then the litmus paper will be bleached and turns white.

Test for carbon dioxide:
— When carbon dioxide is shaken or bubbled through an aqueous solution of calcium hydroxide aka limewater, the limewater turns cloudy.

Question 8

How do you test for cations using flame tests?

Answer 8

Dip a nichrome wire loop into dilute hydrochloric acid the hold it in a blue bunsen flame till it burns without colour Dip the loop into the sample and put it back into the flame.

The flame colours are as follows:

• – Lithium: crimson
• – Potassium: lilac
• – Copper: green
• – Sodium: yellow
• – Calcium: orange-red

Question 9

How do you test for metals that form precipitates?

Answer 9

Add sodium hydroxide solution to your sample.

The colours of precipitates are:

• – Calcium: white
• – Copper II: blue
• – Iron II: green
• – Iron III: brown
• – Aluminium: white then colourless
• – Magnesium: white

NOTE: refer to another source for writing the balanced equations

Question 10

What are the three tests to identify anions?

Answer 10

To test for carbonates:
— They react with dilute acid and produce carbon dioxide. Can then be identified using limewater.

To test for sulfates:

• – Add a few drops of dilute hydrochloric acid followed by barium chloride solution.
• – If sulfate ions are present, a white precipitate of barium sulfate will be formed.

To test for halides:

• – Add a few drops of dilute nitric acid followed by a few drops of silver nitrate solution.
• – Chloride ions form a white precipitate
• – Bromide ions form a cream precipitate
• – Iodide ions form a yellow precipitate

Question 11

What are advantages of using instrumental methods?

Answer 11

Advantages:

• – Very fast
• – Very accurate
• – Very sensitive