Chemical analysis

Question 1

What is a pure substance?

Give some examples

Answer 1

It is a single element or compound that is not mixed with any other substance.

Diamond (carbon)
Oxygen
Glucose

Question 2

What is a mixture?

Give some examples

Answer 2

A mixture contains more than one substance
Milk is a mixture containing water, animal fats, emulsifiers and minerals
Orange juice is a mixture containing water and citric acid molecules
Air is a mixture containing oxygen, nitrogen, argon and carbon dioxide atoms

Question 3

How does the melting and boiling point of a mixture change from a pure substance?

Answer 3

Pure substances have melting and boiling points which are specific temperatures. For example water freezes at 0 and boils at 100 degrees C.
Adding salt makes it a mixture, this lowers the freezing point to between -5 to -10 degrees C. Salt water boils between 101 and 103 degrees C.

Question 4

What is a formulation?

Answer 4

A formulation is a mixture which has been designed as a useful product.

Question 5

Identify some formulations

Answer 5

1. Alloys - mixtures of metals which are harder than pure metals eg steel,
2. Fertilizers - mixtures of compounds such as ammonium nitrate, phosphorus oxide and potassium oxide designed to provide nutrients for plants
3. Fuels - such as petrol and diesel are mixtures of hydrocarbons designed to burn well to power vehicles
4. Medicines - specific mixtures of products containing the active ingredient and binding agents or malitol liquids.
5. Cleaning agents - mixtures of detergents, bleaches, alkalis and sometimes even sodium hydroxide.
6. Foods - Many foods are mixtures of ingredients eg tomato ketchup would include tomatoes, vinegar, spices, sweeteners, salt etc.
7. Paints - mixtures which are either water based or oil based solvents, pigments and binders.

Question 6

What is chromatography?

Answer 6

Chromatography is used to separate mixtures which allows you to identify substances.
There are 4 different types
-paper
- gas
- thin layer
- column

Question 7

Describe the method for using paper chromatography for analysing substances.

Answer 7

1. Draw a line in pencil just above the bottom of the paper, pencil is used as it does not dissolve in the solvent
2. Place a small amount of each of the substances in spots on the pencil line
3. The paper is hung in a beaker of solvent making sure that the pencil line is above the top of the solvent (prevents the substances dissolving in solvent)
4. Allow the solvent to soak into and move up the paper.
5. Take the paper out of the solvent when it gets near to the top of the paper and mark the solvent front- level where the solvent reached
6. The paper is left to dry.

Question 8

Explain how paper chromotography can be used to analyse pure substances and mixtures.

Answer 8

Each substance moves differently depending on its relative attraction to the paper and the solvent.
Substances with a stronger substance to the solvent travel further and more quickly whereas those with a stronger attraction to the paper travel more slowly and not as far.
A pure substance can only produce one spot, whereas a mixture will usually appear as more than one spot. Occasionally the two spots may be on top of each other as they have traveled the same distance so may appear to only be one.
The Rf value can be used to identify substances.

Question 9

Explain how Rf values in paper chromatography are calculated

Answer 9

The Rf value is the ration of the distances travelled by the substance and the solvent. This is why marking the final position of the solvent before it dries is so important.

1. On the chromatography paper measure the distance from the pencil line to the final position of the solvent.
2. Then measure the distance from the pencil line to the centre of a spot.

distance travelled by the substance
Rf = ——————————————————-
distance travelled by the solvent

Question 10

Explain how Rf values in paper chromatography are calculated

Answer 10

The Rf value is the ration of the distances travelled by the substance and the solvent. This is why marking the final position of the solvent before it dries is so important.
Each substance has a different Rf value in different solvents
1. On the chromatography paper measure the distance from the pencil line to the final position of the solvent.
2. Then measure the distance from the pencil line to the centre of a spot.

     distance travelled by the substance Rf =   -------------------------------------------------------
     distance travelled by the solvent Rf value is always < 1

Question 11

Describe the test for oxygen

Answer 11

A glowing splint inserted into a test tube of oxygen will relight

Question 12

Describe the test for hydrogen

Answer 12

A burning splint inserted into a test tube of hydrogen will burn rapidly and cause a popping sound

Question 13

Describe the test for chlorine gas

Answer 13

DAMP litmus paper inserted into the gas will become bleached white

Question 14

Describe the test for carbon dioxide

Answer 14

If carbon dioxide is bubbled through lime water (calcium hydroxide solution) it turns the limewater cloudy.

Question 15

What is effervescence?

Answer 15

The production of bubbles of gas during a reaction

Question 16

What types of observation might you see during reactions.

Answer 16

1. Colour change - reaction has taken place
2. Effervescence - gas is produced
3. Temperature change - reactions may be exothermic or endothermic
4. Precipitate produced - state the colour
5. Solubility of solids - whether a solute dissolves in a solvent or not.
6. Solubility of liquids - whether liquids are miscible (mix) or non miscible (don’t mix)