Atoms, Elements and Compounds
Atoms- All substances are made of atoms (very small particles). Each atom has a small central nucleus made up of protons and neutrons that is surrounded by electrons.
Elements- A substance which contains only one sort of atom is called an element. There are about 100 different elements.
The atoms of each element are represented by a different chemical symbol, for example, O for oxygen, Na for sodium, C for carbon, and Fe for iron. These symbols are all contained in the periodic table. The groups in the periodic table contain elements that have a similar properties.
Compounds- Compounds are substances in which the atoms of two or more elements are chemically combined, i.e. the atoms are held together by chemical bonds (not just mixed together).
When elements react, the atoms can form chemical bonds by... sharing electrons or giving or taking electrons.
Chemical Reactions and Writing Balanced Equations
Chemical Reactions- You can show what has happened during a reaction by writing a word equation with the substances that react (the reactants) on one side of the equation and the new substances formed (the products) on the other.
The total mass of the products of a chemical reaction is always equal to the total mass of the reactants.
This is because the products of a chemical reaction are made up from exactly the same atoms as the reactant - no atoms are lost or made!
That means chemical symbols equations must always be balanced: there must be the same number of atoms of each element on the reactant side of the equation as there is on the product side.
Writing Balanced Equations- follow these steps to write a balanced equation:
1.write a word equation for the chemical reaction
2.substitute in formulae for the elements or compounds involved
3.balance the equation by adding numbers in front of the reactants and/or products
4.write down a balanced symbol equation.
Limestone is a sedimentary rock which consists mainly of the compound calcium carbonate. It is cheap, easy to obtain and has many uses.
Building Material- Limestone can be quarried and cut into blocks, and used to build walls of houses in regions where it is plentiful. Over time it can be eroded by acid rain, but this is a very slow process.
Neutralising Agent- Alkali's in soil can be 'washed out' by acid rain. Excess acidity in soils can cause crop failure. Powdered limestone can correct this but it works quite slowly. When calcium carbonate is heated in a kiln it decomposes. This reaction is called thermal decomposition and it causes the calcium carbonate to break down into calcium oxide (quicklime) and carbon dioxide. Carbonates of other metals decompose in a similar way. The calcium oxide (quicklime) can then be reacted (slaked) with water to produce calcium hydroxide (slaked lime). Calcium hydroxide (like all hydroxides), is a strong alkali. It can be used to neutralise soils and lakes much faster than powdered limestone.
calcium carbonate > heat > calcium oxide + carbon dioxide
Glass- Glass is made by mixing limestone, sand and soda (sodium carbonate) and heating the mixture until it melts. When cool, it is transparent.
limestone + sand + soda > heat > glass
Cement, Mortar and Concrete- Powdered limestone and powdered clay are roasted in a rotary kiln to produce dry cement.
When the cement is mixed with sand and water is produces mortar, which is used to hold during building.
When the cement is mixed with water, sand and gravel (crushed rock) a slow reaction takes place where a hard, stone-like building material, called concrete, is produced.
Ores, Extracting Metals from their Ores and Iron
Ores- The Earth's crust contains many naturally occurring elements and compounds called minerals. A metal ore is a mineral or mixture of minerals from which economically viable amounts of pure metal can be extracted. This can change over time.
Extracting Metals from their Ores- The method of extraction depends on how reactive the metal is. Unreactive metals like gold exist naturally. They are obtained through physical processes such as panning.
Most metals are found as metal oxides or compounds that can be easily changed into a metal oxide. To extract a metal from its oxide the oxygen must be removed by heating the oxide with another element in a chemical reaction. This process is called reduction.
Metals that are less reactive than carbon can be extracted from their oxides by heatin them with carbon. (The carbon is a more reactive element, so it will displace the metal and form a compound with the oxygen.)
Iron- Iron oxide can be reduced in a blast furnace to produce iron.
Molten ironobtained from a blast furnace contains roughly 96% iron and 4% carbon and other metals. Because it is impure the iron is very brittle with limited uses. To produce pure iron, all the impurities would have to be removed.
The atoms in pure iron are arranged in layers, which can slide over each other easily. This makes pure iron soft and malleable - it can be easily shaped. However, it it too soft for many practical uses.
The properties of iron can be changed by mixing it with small quantities of carbon or other metals to make steel, which is an alloy.