Teacher's notes and answers to questions in the book - Hodder Plus ...

WJEC GCSE Additional Science Teacher’s Notes
_ Sodium chloride (common salt) – an ionic compound _
(pages 100–102)
Questions
6. How are sodium and chloride ions formed from sodium and chlorine atoms
Sodium ions form from sodium atoms when the outermost electron on the sodium atom is
removed. Chloride ions form from chlorine atoms when an extra electron is added to its
electron configuration.
7. Why is sodium chloride (common salt) used as a flavouring and preservative for food
Our tongues have evolved to sense the flavour of common salt ‘saltiness’ (along with
bitterness, sweetness and sourness). Common salt is used as a food preservative, particularly
with meat or fish, because it draws the water moisture out of the food (and the microorganisms
that cause the meat to go rotten) allowing the food to be edible for a longer period.
8. Explain why sodium chloride crystals are brittle.
Sodium chloride crystals are brittle because when a stress is applied across the crystal the ion
layers will shift causing a fracture.
9. What happens when sodium chloride crystals are added to water
When added to water, the sodium chloride crystal lattice breaks down (dissolves) and a
solution of positive sodium ions and negative chloride ions is formed.
10. Molten and solid sodium chloride contain the same sodium and chloride ions. Why does molten sodium
chloride conduct electricity, while solid sodium chloride does not conduct electricity.
In solid sodium chloride the ions are not free to move, yet in molten sodium chloride they are
free to move (and create an electric current).
11. Calcium oxide is another ionic compound. Calcium has two electrons in its outer shell, and oxygen has
six electrons in its outer shell. Draw dot and cross diagrams to show how calcium oxide is formed by
electron transfer from calcium to oxygen atoms.
calcium ion, Ca 2+ , 2,8,8 oxygen ion, O 2– , 2,8
12. Draw dot and cross diagrams to show the formation of:
a
Lithium fluoride from
lithium and fluorine
lithium ion, Li + , 2,8
fluoride ion, F – , 2,8
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