Atoms and Elements
An element is a pure substance containing one type of atom and cannot be split up into anything simpler by chemical methods. An atom is made up from even smaller particles called protons, electrons and neutrons and can be split up by physical means but not by chemical reaction.
All substances are made up from one or more of the 109 elements. These elements combined together in different ways to give all of the substances on Earth. The elements are listed in the periodic table.
In the periodic table, a Group is a vertical column of elements with very similar properties. Some Groups have names:
Group 1: The Alkali Metals
Group 2: The Alkaline Earth Metals
Group 3: The Halogens
Group 4: The Noble Gases
A Period is a horizontal row ofelements and there's always a gradual change or trend in properties across a Period.
Most of the known elements are solids and metals. There are only two liquid elements at room temperature and atmospheric pressure: bromine is a liquid non-metal and mercury is a liquid metal. The elements that are gases at room temperature and atmospheric pressure are hydrogen, helium, nitrogen, oxygen, fluorine, neon, chloride, argon, krypton, xenon and radon.
Mixtures
Mixtures are substances not chemically bonded. They are formed by elements or compounds 'mixed' together. A mixture has the properties of the elements/compounds that made up the mixture. Examples are sea water and air which are made up of different particles not combined. These particles can be separated using one of the four physical separation techniques: Filtration, Evaporation, Chromatography or Distillation. Each method makes use of the different properties of the mixture's parts to separate them. Filtration is used to separate an insoluable solid from a solution, whereas evaporation is used to obtain the solute from the solution. Distillation is used to obtain solvent from a solution or to separate mixtures of liquids. Chromatography is used to separate mixtures of substances dissolved in a solvent.
Separating Mixtures
Filtration and Evaporation are used to obatin salt from Rock Salt (a mixture of salt and impurities such as sand). The method is based on the fact that salt is soluble and impurities are insoluble. The Rock Salt is grind into small pieces and added with water to allow the salt to dissolve. This is then filtered to separate the impurities from the salt solution and the filtered solution is called the filtrate. The solution is then evaporated to remove the water and the salt forms as crystals via crystallisation. Good crystals will not be produced if heat is used to evaporate all the water to dryness. To produce good crystals, the solution should be evaporated until a small volume of solution remains and then left to cool for the crystals to form.
Simple distillation is used to obtain solvent from a solution. It is evaporation followed by condensation and utilises the fact that solute and solvent have different boiling points. The solution is heated in a flask to the boiling point of the solvent causing the solvent to evaporate out of the flask into the condenser where cooling takes place to condense the vapour back to liquid.
Fractional distillation is used to separate mixture of miscible liquids if the boiling points of the liquids are not too close together. For example, a mixture of ethanol (boiling point 78°C) and water is first heated slowly where ethanol starts to boil and the vapour passes up the fractional distillation column. Any water vapour which gets into the column at this stage condenses and drops back into the flask and only ethanol distils over. The liquid collected is called the first fraction and consists almost entirely of ethanol. At temperatures between 80°C - 95°C (above ethanol's boiling point but below water's boiling point), a second fraction is collected consists of a mixture of ethanol and water. Above 95°C, the third fraction collected will consist mostly of water. It is generally difficuly to separate completely a mixture of miscible liquids by fractional distillation. Fractional distillation is used in industry to:
- separate oxygen and nitrogen from air by fractional distillation of liquid air
- refine petroleum to produce valuable products
- concentrate ethanolin whisky production
Chromatography is often used to separate mixtures of inks or dyes. It relies upon the different rates at which the dyes spread across a piece of filter paper. Sample spots are put on the 'base line' drawn on a square filter paper and the paper is coiled into a cylinder and put into a tank with a small amount of solvent at the bottom. The solvent travels up the filter paper and the spots are separated. Different dyes in the spots will stop at different positions allowing identification to be made.
The applications of chromatography include medical and food diagnostic. The dyes in food can be identify to ensure they are permitted dyes. Chromatography of urine can detect excess of amino acids leaving the body indicating the person is unable to use the amino acids to build up proteins.
Compounds
Compounds contain two or more elements chemically bonded together. The particles in a compound are called molecules, formed when different atoms bond. Compounds are produced by chemical reactions and are new substances with properties totally different from the original elements. For example, in a mixture of iron and sulphur, iron can be separated using a magnet because iron is magnetic. However, when headed the iron reacts with sulphur to form iron sulphide which is not magnetic:
Iron + Sulphur --heat--> Iron Sulphide
Compounds can be split up back into their original elements but needs a large amount of energy to reverse the reaction.
A compound has a fixed composition by mass. Irrespective of how black copper oxide is formed, the mass of the copper in the compound is always four times the mass of oxygen.
When two elements combine the ending is usually "-ide": Magnesium Oxide contains 2 elements (magnesium and oxygen). However, there are exceptions, and Sodium hydroxide is composed of three elements sodium, oxygen and hydrogen.
When three or more elements combine and one of them is oxygen, the ending is"-ate":
Copper Sulphate contains copper, sulphur and oxygen
Calcium carbonate contains calcium, carbon and oxygen
Sodium hydrogensulphate contains sodium, hydrogen, surphur and oxygen
Compound Structure
A compound may compose of small molecules of atoms or gaint structures of atoms or ions. The structure of the compound determines the electrical conductivity and heating properties of the compound.
- Heating. A compound composed of small molecules will have low melting and boiling points. A gaint structure of atoms and ions will have high melting and boiling points.
- Electrical conductivity. On melting, a giant structure of atoms will not conduct electricity but a gaint structure of ions will.
Two examples of compounds with different structures are methane (CH4) and silicon dioxide (SiO2). Methane is made up of molecules each containing one carbon atom combined with four hydrogen atoms. Silicon dioxide is a solid with a high melting point - it has a gaint structure of atoms. Silicon dioxide is also known as silica and sand.