Chemistry C5

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Bonding:

Covalent bonds are when atoms in a molecule SHARE electrons. Some examples are carbon dioxide, oxygen and hydrogen. These molecules have weak forces of atrraction between others so they have a low BP and MP. They cannot conduct electicity because there are no free electrons. They are usually gases or liquids at room temperature.

Ionic bonds are when atoms in a molecule permanently TAKE or LOSE electrons from the other. They are made from ions (charged particles). Ions with opposite charges are attracted to each-other. They form a giant ionic lattice where there are strong ionic bonds between all the ions. One giant ionic lattice make up one salt crystal. As there are strong forces of attraction between the ions, a lot of energy is needed to break the compound; it has a high BP and MP. When dissolved or molten, the ions are separate so there are free electrons. Therefore they can conduct electricity because they are free to move. They are usually solids at room temperature.

In hydrosphere - water contains many dissolved ionic compounds (salts). Some examples of salts are sodium chloride, manesium chloride and potassium bromide.

In lithosphere - carbon forms giant covalent structures (diamond, graphite), silicon dioxide is also a giant covalent structure.

Diamond is a large covalent structure where each carbon atom forms 4 covalent bonds with each-other. This makes it very rigid and hard. The strong covealent bonds make it harder to break so it has a higher MP. It doesn't conduct electricity because there are NO free electrons, not even when it is molten.

Graphite is a loarge covalent strucure where each carbon atom forms 3 covalent bonds with each-other. This creates a sheet of carbon atoms, which are free to slide over each-other. This mean that there are lots of free electrons so it can conduct electricity. Because of the loose layers, it can be rubbed off on paper to leave a black mark. Even though the covalent bonds aren't as strong as in diamonds, graphite still has a high MP.

Silicon Dioxide (silica) consists of silicon and oxygen (mainly found in the Earth's crust). It has a similar structure to diamonds e.g. it has a high MP and doesn't conduct electricity. Each silicon atom is bonded to 4 oxygen atoms. Each oxygen atom is bonded to 2 silicon atoms. It is mainly found in sand.

Identifying positive ions:

-Flame tests can be used to identify different ions present in a compound. Some metals give a characteristic flame colour when heated.

Sodium = orange/yellow flame

Potassium = lilac flame

Calcium = dark red flame

Copper = blue/green flame 

-Sodium hydroxide can be added to the compound to produce an insoluble solid (precipitate). Many metal hydroxide are insoluble. The colour of the precipitate formed can help to identify the metal ion present in the compound.

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