Chemical analysis

Question 1

Define a pure substance and identify pure substances and mixtures from data about melting and boiling points

Answer 1

Pure substance - a substance only containing one element or compound

If a substance is pure they will melt and boil at specific temperatures. If impure the melting point will lower and increase melting range. The boiling point will increase and may also increase boiling range.

Question 2

Describe a formulation and identify formulations given appropriate information

Answer 2

Formulation - useful mixtures with a precise purpose.

Question 3

Describe chromatography, including the terms stationary phase and mobile phase and identify pure substances using paper chromatography

Answer 3

Chromatography - analytic method uses to separate the substances in a mixture. It can be used to identify substances.

Stationary phase - where molecules can’t move (e.g. chromatography paper).

Mobile phase - where molecules can move (the solvent e.g. water/ethanol).

A pure substance will only ever form one spot in any solvent.

Question 4

Explain what the Rf value of a compound represents, how the Rf value differs in different solvents and interpret and determine Rf values from chromatograms

Answer 4

Rf value - the ratio between the distance travelled by the dissolved substance and the distance travelled by the solvent.

Question 5

Required practical 12: investigate how paper chromatography can be used to separate and tell the difference between coloured substances (inc calculation of Rf values)

Question 6

Explain how to test for the presence of hydrogen, oxygen, carbon dioxide and chlorine