Q7.

(a) The graph shows how a sample of barium-143, a radioactive isotope with a short half-life, decays with time.

(i) What is meant by the term isotope?

Isotopes of an element have equal number of protons but different number of neutrons

Isotopes of an element have the same atomic/proton number but a different mass/nucleon number

(1 mark)

(ii) What is meant by the term half-life?

Half-life is the time taken for activity (or 'count rate' or 'number of nuclei') to decrease to half.

The examiner would not accept the time taken for radioactivity/substance/ material to halve! It is best to learn by heart a good definition for terms such as half life so that you express the idea very clearly in an examination.

(1 mark)

(iii) Use the graph to find the half-life of barium-143.

12 seconds

It is always a good idea to work it out twice - to double check.... see the graph - the time for the count to halve from 4000 is just the same as the time for it to alve from 2000.

(1 mark)

(b) Humans take in the radioactive isotope carbon-14 from their food. After their death, the proportion of carbon-14 in their bones can be used to tell how long it is since they died. Carbon-14 has a half-life of 5700 years.

(i) A bone in a living human contains 80 units of carbon-14. An identical bone taken from a skeleton found in an ancient burial ground contains 5 units of carbon-14.

Calculate the age of the skeleton.

80 to 40 - First half life

40 to 20 - Second half life

20 to 10 - Third half life

10 to 5 - Fourth half life

4 half lives = 4 x 5700 = 22800 years

(2 marks)

(ii) Why is carbon-14 unsuitable for dating a skeleton believed to be about 150 years old?

The decay of carbon-14 over 150 years is insignificant there is very little decay the change is too small

(1 mark)

Some people suggest that radioactive liquid waste can be mixed with water and then safely dumped at sea.

Do you agree with this suggestion?

Either argument gains full credit or 3 marks - there is not a 'right' or 'wrong' answer here.

FOR

massive dilution of waste within the ocean

reduces concentration (within a given volume) to insignificant levels

dumped distant from human habitation

AGAINST

pollution (of the sea/beach)

mutation or harm caused to living things (animals/plants)

effect on food chain

long period of time necessary for radioactive isotopes to be harmless

(3 marks)

(Total 9 marks)