C5 - Chemistry - OCR 21st Century Science

Notes for C5

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C5 ­ Chemicals of the Natural Environment ­ Answers
1. The air contains a number of gases. The amount of water vapour in the air varies from place to place, and day to day. For this
reason, the proportions of the gases in the air are usually given for dry air. Some of the gases in the air are elements: nitrogen
(N2), oxygen (O2) and argon (Ar). But carbon dioxide (CO2) is a compound (as is and water vapour, H2O).
2. Most non-metal elements are molecular, and most of these consist of molecules with just two atoms joined together.
Examples include nitrogen (N2), oxygen (O2), chlorine (Cl2) and bromine (Br2). Most compounds between non-metal elements
are also molecular. Some examples are: water (H2O), carbon dioxide (CO2), nitrogen dioxide (NO2) and ammonia (NH3).
3. All of these substances have very strong covalent bonds between the atoms, but much weaker forces holding the molecules
together. When one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong
covalent bonds. Because the weak intermolecular forces break down easily these substances have low melting and boiling
points. This means simple molecular substances are gases, liquids or solids with low melting points, and low boiling points.
4. Simple molecular substances do not conduct electricity. This is because they do not have any free electrons, and the
molecules do not have electric charges.
5. In a covalent bond, the electrons are shared between the atoms. This strong, electrostatic
attraction is caused between the positive nuclei and shared pair of negative electrons. Some
molecules have a double covalent bond, meaning they have two shared pairs of electrons.
Others have a triple covalent bond, meaning they have three shared pairs of electrons.
Covalent bonds are strong, and a lot of energy is needed to break them. For example each
carbon dioxide molecule has a carbon atom joined by four covalent bonds to two oxygen atoms, which have two covalent
bonds each.
7. The Earth's hydrosphere is the oceans. They consist mainly of water, with some dissolved compounds. The ionic compounds
that are dissolved in sea water are mainly salts, and make the water taste salty.
8. The ions in a compound such as sodium chloride are arranged in a lattice structure. This regular arrangement results in the
formation of a crystal. This pattern is repeated in all directions, giving a giant three-dimensional lattice structure in sodium
chloride crystals. Because of the strong electrostatic forces between them, it takes a great deal of energy to separate the
positive and negative ions in a crystal lattice. This means that ionic compounds have high melting points and boiling points.
9. Solid ionic compounds do not conduct electricity, because the ions are held firmly in place. They cannot move to conduct the
electric current. But when an ionic compound melts, the charged ions are fee to move. Molten ionic compounds do conduct.
When a crystal of an ionic compound dissolves in water, the ions separate. Again, the ions are free to move, so a solution of
an ionic compound in water also conducts electricity.
10. Ionic Bonding occurs between a metal and a non-metal. It is where electrons are transferred
from one atom to another to create a full outer shell of electrons on each atom. By transferring
electrons the atoms become ions - atoms with a charge. An atom will have a positive charge if it
loses electrons and a negative charge if it gains electrons.
For example, a sodium atom gives an electron to a chlorine atom. The result is a sodium
ion (2,8)+ and a chloride ion (2,8,8)-. Both ions have full outer shells.
11. Sea water contains a number of dissolved salts. When these salts dissolve in water, the ions
separate. Sea water therefore contains a mixture of ions. The most common ions in sea water
are shown in the table: It is possible to deduce from the table which salts dissolved to form the
mixture of ions:
When sodium chloride dissolves it forms sodium ions and chloride ions.
When magnesium chloride dissolves it forms magnesium and chloride ions.

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When magnesium sulfate dissolves if forms magnesium ions and sulfate ions.
When potassium chloride dissolves it forms potassium ions and chloride ions.
When potassium bromide dissolves it forms potassium ions and bromide ions.
Magnesium ions each have two positive charges. Chloride ions each have a single negative charge. For the charges to
cancel out in the neutral salt magnesium chloride, they must be in a ration of 1:2. So the formula of magnesium chloride is
12.…read more

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Attached to this are the four bases adenine, guanine, cytosine and thymine. The
sequence of these bases comprises the genetic code.
22. Almost all molecules in living things contain carbon. Carbon moves in a cyclical way, passing from one organism to another in
the biosphere, and between the other spheres of the environment (lithosphere, hydrosphere and atmosphere). The carbon
cycle is the key factor in maintaining the balance of carbon dioxide in the air.
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Instead, it is dissolved in molten cryolite, an aluminium compound with a lower melting point than
aluminium oxide. The use of cryolite reduces some of the energy costs involved in extracting aluminium.
Aluminium metal forms at the negative electrode and sinks to the bottom of the tank, where it is tapped off
Oxygen forms at the positive electrodes. This oxygen reacts with the carbon of the positive electrodes, forming
carbon dioxide, and they gradually burn away.…read more


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