Experimental Chemistry

Question 1

Define compound

Answer 1

Pure substances made up of atoms of two or more elements chemically combined together

Question 2

Define mixture

Answer 2

Different substances not chemically combined together

Question 3

Separation techniques for mixtures

Answer 3

• Sublimation
• Filtration
• Evaporation to dryness
• Crystallisation
• Simple distillation
• Fractional distillation
• Paper chromatography
• Separating funnel

Question 4

Why do substances in a mixture retain their respective properties?

Answer 4

The formation of a mixture does not involve a chemical reaction. Hence, no chemical change is taking place allowing each substance to retain it’s respective properties.

Question 5

Differences between compound and mixture

Answer 5

• Compound
- separated by chemical methods
- fixed melting boiling point
- fixed composition
- has its own properties which are different from its constituent elements

• Mixture
- separated by physical methods
- wide range of melting boiling points
- no fixed composition
- same properties as its constituent elements

Question 6

Define a pure substance

Answer 6

Contains only one type of substance ( element or compound )

Question 7

What are the positions of pure/impure substances on chromatogram?

Answer 7

• Pure substance: one spot
• Impure substance: more than one spot

Question 8

Importance of making pure substances

Answer 8

• medicinals drugs to avoid undesirable effects such as food poisoning from impurities
• food and beverage industry so that food and drink products are safe for public consumption and free from impurities that can affect human health

Question 9

Define sublimation

Answer 9

When a solid substance changes into gas directly without melting

Question 10

Crystallisation procedure

Answer 10

1. Add excess copper (ii) oxide to fixed volume of HCl
   { ensures all acid used up }
2. Filter copper (ii) chloride solution { removes excess copper (ii) oxide }
3. Heat filtrate until saturated
   { allow crystals to form easily }
4. Allow solution to cool slowly for bigger copper (ii) chloride crystals to form
5. Wash crystals with water and dry them

Question 11

Advantages of chromatography

Answer 11

• quick and accurate analysis of sample
• requires only small amount of sample
• separates complex mixtures

Question 12

Rf value

Answer 12

Distance travelled by spot
—————————————
Distance travelled by solvent

Question 13

Use of locating agent
(Ninhydrin/Ultraviolet light)

Answer 13

Make invisible components on chromatogram visible

Question 14

Uses of Paper Chromatography

Answer 14

• medical industry to diagnose illness
• food industry to monitor use of artificial flavourings and colourings
• forensic science to investigate evidence of crime