Revision on Common Misconceptions in Chemistry

Solid sodium chloride does not conduct electricity but a solution of sodium chloride is a good conductor. Explain why.

• In solid state, sodium and chloride ions are held in fixed positions but when dissolved in water, there are free moving electrons that can carry charges and thus conduct electricity.

• In solid state, sodium and chloride ions are held in fixed positions but when dissolved in water, there are delocalised electrons that can carry charges and thus conduct electricity.

• In solid state, sodium and chloride ions are held in fixed positions but when dissolved in water, there are free moving ions that can carry charges and thus conduct electricity.

• In solid state, sodium and chloride ions are held in fixed positions but when dissolved in water, there are mobile ions that can carry charges and thus conduct electricity.

Calcium reacts with chlorine to form a compound, calcium chloride (CaCl2). Explain with reference to structure and bonding why calcium chloride has a high melting point.

• Calcium chloride has ionic bonding. The ions are held by strong electrostatic forces of attraction which require a lot of energy to overcome.

• Calcium chloride has an ionic bond. The ions are held by strong electrostatic forces of attraction which require a lot of energy to overcome.

• Calcium chloride has a crystal lattice structure. The ions are held by strong electrostatic forces of attraction which require a lot of energy to overcome.

• Calcium chloride has a giant lattice structure. The ions are held by strong electrostatic forces of attraction which require a lot of energy to break.

Answer explanation

1) We do not usually mention bonds in Ionic Compound. Instead, we should always give the answer with keywords such as “electrostatic forces of attraction between the ions”.

2) Specific mention must be made to describe structure : “Crystal Lattice Structure”. Other accepted keywords are “Giant Ionic Structure” or “Giant Lattice Structure”.

3) "Break" is the wrong term to use for "electrostatic forces of attraction". The only acceptable term is "overcome".

The symbols for two isotopes of hydrogen are shown. Using the information given, explain why Hydrogen-1 and Hydrogen-2 are considered isotopes of hydrogen.

• Both Hydrogen-1 and Hydrogen-2 contains same number of protons but different number of neutrons.

• Both contain 1 proton but different number of nucleons.

• Both contain 1 neutron. Hydrogen-1 contains 0 proton but Hydrogen-2 contains 1 proton.

• Both contain 1 proton. Hydrogen-1 contains 0 neutron but Hydrogen-2 contains 1 neutron.

Answer explanation

1) Stating of specific number of protons and neutrons is required according to question " Using the information given..."

2) Nucleons refer to the sum of both protons and neutrons.

Some cars have components made from an alloy of 90% proportion by mass of aluminium and 10% proportion by mass of silicon. Explain why this alloy is stronger than pure aluminium.

• Molecules of different elements have different sizes. This prevent layers of atoms from sliding over one another easily when a force is being applied.

• Atoms of different elements have different sizes. This disrupts the orderly arrangement of atoms in pure aluminium and prevents layers of atoms from sliding over one another easily when a force is being applied.

• Atoms of different elements have different sizes. This disrupts the orderly arrangement of atoms in pure aluminium. This prevents layers of atoms from sliding over one another easily.

• Atoms of Silicon have different sizes from aluminium atoms which disrupt the regular arrangement of atoms in pure Aluminium. This prevent layers of atoms from sliding over one another easily when a force is being applied.

Answer explanation

1) There must be mention of "when a force is applied" while describing layers of atoms sliding over each other since atoms do not slide on their own without a force applied.

In the control of rusting, an iron object can be coated with a layer of zinc. Explain how this method helps in rust protection.

• Zinc is more reactive than iron and will rust in place of iron, acting as a sacrificial metal.

• Zinc is more reactive than iron and will react more readily with oxygen and water, acting as a sacrificial metal.

• Zinc is more reactive than iron and will corrode in place of iron, acting as a sacrificial metal.

• Zinc is more reactive than iron and loses electrons more readily, acting as a sacrificial metal.

Answer explanation

The term "rust" is only applicable for the metal iron.

The decomposition of copper(I)bromide is a disproportionation reaction. Explain why in terms of oxidation states.

2CuBr -->CuBr₂ + Cu

• Cu has been oxidised as its oxidation state increased from +1 to +2. Cu has been reduced as its oxidation state decreased from +1 to 0.

• Cu has been oxidised as it loses 1 electron in Cu⁺ to form Cu²⁺ in CuBr₂.

  Cu has been reduced as it gains 1 electron in Cu⁺ to form Cu.

• Cu has been oxidised as its oxidation state increased from +1 in CuBr to +2 in CuBr₂. Cu has been reduced as its oxidation state decreased from +1 in CuBr to 0 in Cu.

Answer explanation

1) Specific mention must be made to the before and after oxidation states in the respective particles.

2) Question only wants explanation using oxidation states.

The walls of the catalytic converter are made from silicon carbide, SiC. SiC is a hard solid that will not melt at the high temperatures of the exhaust. The structure of SiC is shown.

Suggest how the structure and bonding in silicon carbide makes it resistant to melting, even at high temperatures.

• SiC has a tetrahedral structure. A large amount of energy is required to break the strong covalent bonds between Si and C atoms.

• SiC has a giant covalent structure. A large amount of force is required to break the strong covalent bonds between Si and C atoms.

• SiC has giant molecular structure. A large amount of energy is required to break the strong covalent bonds between Si and C atoms.

• SiC has giant molecular structure. A large amount of energy is required to overcome the strong covalent bonds between Si and C atoms.

Answer explanation

1) Question is about melting/ boiling point, hence a large amount of "energy" rather than "force" should be used.

2) In this example, strong covalent bonds are involved hence the term 'break' is more apt than 'overcome'

3) For structure, only giant covalent or molecular structure are accepted.

Using your understanding of structure and bonding, explain why carbon dioxide does not conduct electricity in any state.

• Carbon dioxide has a simple covalent structure with weak intermolecular forces of attraction between molecules.

• Carbon dioxide has a simple covalent structure consisting of simple molecules. There are no free moving ions in any state to conduct electricity.

• Carbon dioxide has a simple covalent structure consisting of simple molecules. The unbonded electrons are held in fixed positions and cannot conduct electricity.

• Carbon dioxide has a simple covalent structure consisting of simple molecules. There are no free moving electrons or ions in any state to conduct electricity.

Answer explanation

Simple covalent molecules do not contain any ions at all.