Fig. 1 shows a molecular model of iodine. At a coastal laboratory, iodine crystals are warmed gently to make iodine vapour for a test of starch in seaweed s...
Question 1 Report
Fig. 1 shows a molecular model of iodine. At a coastal laboratory, iodine crystals are warmed gently to make iodine vapour for a test of starch in seaweed samples. The crystals change directly to a purple gas without forming a liquid. The scientist compares this change with melting sodium chloride.
(a) Name the type of bonding between the two iodine atoms. [1]
(b) What is the name of the change from solid iodine directly to gas? [1]
(c) Suggest why iodine has a low melting point compared with sodium chloride. [2]
(d) Give the number of iodine atoms in five iodine molecules. [1]
Fig. 1 shows a model of a water molecule. A water company uses activated carbon to remove some dissolved compounds from drinking water. The chemist reminds a student that water itself is a simple molecular substance, whereas the carbon filter has a giant structure. The water is boiled before a sterile sample is taken.
(a) What is the formula of water? [1]
(b) Name the type of bonding inside a water molecule. [1]
(c) Suggest why boiling water does not break the O-H bonds. [2]
(d) Give one property that makes activated carbon useful in a filter. [1]
Answer Details
Iodine
- (a) The iodine atoms share electrons, so the bond is covalent bonding. [1]
- (b) A direct change from solid to gas is sublimation. [1]
- (c) Iodine consists of simple molecules. [1] Only weak intermolecular forces between molecules are overcome on melting, so its melting point is low compared with the strong ionic attractions in sodium chloride. [1]
- (d) Each \(\text{I}_2\) molecule has two iodine atoms: \(5\times2=10\). 10 [1]
Water and activated carbon
- (a) Water is \(\text{H}_2\text{O}\). [1]
- (b) The bonds inside a water molecule are covalent bonds. [1]
- (c) Boiling overcomes weak forces between water molecules. [1] The O-H covalent bonds are much stronger, so they are not broken. [1]
- (d) Activated carbon is useful because it has a large surface area or is porous. [1]
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