Atoms, Elements and Compounds
There is about 100 different elements in which all substances are made from. These are all listed in the periodic table. Each element is made of one type of atom. Atoms are represented by a chemical symbol, eg. Na for an sodium atom and Fe for an iron atom. Atoms all have tiny nucleuses surrounded by eletrons, when elements react their atoms join with other atoms electrons. Atoms for chemical bonds by losing, gaining or sharing electrons.
A compound contains two or more atoms bonded together.
Elements are all made of one type of atom.
The symbols that represent the elements are used to show what happens in chemical equations.
Limestone and its Uses
We quary large amounts of limestone rock becasue it has a lot of uses. Blocks of limestone can be used to make buildings, and its used in making:
- Quicklime (by thermal decompostition of limestone makes quicklime (carbon oxide) and carbon dioxide.)
Limestone is made up mainly of calcium carbonate, CaCO3. When heated strongly, calciun carbonate decomposes to make quicklime ( carbon oxide, CaO) and carbon dioxide (CO2.)
We represent the reaction by the word equation:
Calcium carbonate goes to calcium oxide + carbon dioxide.
This is done on a large scale in lime kilns.
REMEBER: Thermal decompostition means 'breaking down using heat.'
Metal carbonates decomposes in a similar way to calcium carbonate when heated hot enough. A burnsen burner flame is not hot enough to decompose sodium carbonate or potassium carbonate. These reaction can represented by a balanced chemical equation, for example:
CaCO3 goes to CaO + CO2.
- Symbol equations should be balanced with the same number of each type of atom on both sides because you cannot lose atoms in a reaction.
- Atom aren't created or destroyed in chemical reactions. So the mass of the products is always the same as the mass of the reactants.
Carbonates of metals decompose when heated producing metal oxide and carbon dioxide.
Quicklime and Slaked Lime
Quicklime (calcium oxide) reacts with water to produce slaked lime (calcium hydroxide) Quicklime + water = slaked lime.
CaO + H2O = Ca(OH)2
- Calcium hydroxide (slake lime) is only slightly soluble with water, but little a dissolves to form a solution called lime water.
- Calcium hydroxide, (slake lime) reacts with carbon dioxide to form calcium carbonate, which is insouble in water. This is why carbon dioxide bubbled into lime water makes it go cloudy. (Ca(OH)2 + CO2 = CaCO3 + H2O)
- Mortar is used to hold stone or bricks together in buildings. Lime mortar is made by mixing slaked lime with sand and adding water.
- When carbon dioxide in the air reacts with carbon hydroxide (slake lime) in the mortar it forms calcium carbonate and sets hard.
(Ca(OH)2 + CO2 = CaCO2 + H2O)
Lime water is used as a test for carbon dioxide, as carbon dioxide makes the water go cloudy.
Cement, Concrete and Glass
To make cement, limestone is mixed with clay then heated strongly and the product is powered. Mortar made with cement and sand is stronger and sets faster than lime mortar, it will even set in the rain and underwater.
A mixture of cement, sand, stones or crushed rock and water is called 'concrete'. This can be poured into moulds or spread out before it sets to produce different shapes. Concrete can be reinforced by pouring it around steel.
Glass is used to allow light into buildings and make them waterproof. It is made from limestone, sand and sodium carbonate.
The properties of cement, glass and concrete can be modified for specific uses. This can be done by using different proportions of the main ingredients or by adding other substances.