|
Chemistry - Historical models of the atom As scientists discovered new evidence, the scientific model of the atom was updated or replaced. 1)An early model Atoms were originally thought to be tiny spheres that could not be made smaller. 2 Plum pudding model Electrons were discovered. negative electron The atom is a positively charged sphere containing electrons. The alpha particle-scattering experiment showed that most positive alpha particles fired at an atom went through the atom. 3) Nuclear model Atom’s mass is mainly a positive central nucleus 4) Niel Bohr’s model Bohr's calculations agreed with experiments showing the electron orbits. small, positively charged sphere experiment showed that the positive nucleus was made of small, positively charged particles, which were given the name 'protons'. 5) Atomic model with protons 6) Atomic model with neutrons James Chadwick's experiments showed, about 20 years after the nucleus had been accepted, that the nucleus also contained particles with no charge; these were given the name neutron 
0 Comments
Chemistry -Distillation Distillation is used to separate liquid from a mixture. Simple distillation The condenser has two tubes, one inside the other. Cold water runs through the space between the two tubes, keeping the condenser cold. The cooling water does not mix with the substance being separated. In a mixture of two liquids, the liquid with the lower boiling point is collected as the distillate. In a solution of a solid in water, the water is collected as the distillate.  Fractional Distillation Fractional distillation is used to separate a mixture of more than two liquids using the laboratory apparatus shown. The column is packed with glass beads. The different liquids leave the top of the column in order of increasing boiling point.  Filtration, crystallisation and chromatography Mixtures are separated by physical processes. Separation of mixtures does not require chemical reaccions and no new substances are made. Filtration filter paper evaporating basin filtrate The mixture is poured into the tunnel. 1. Insoluble solids remain in the filter paper. 2. Solutions and liquids go through into the flask.  Filtration Crystallisation The solution is heated to concentrate the solution, and crystals may start to form. It is then left in a warm place for the remaining water to evaporate Crystals form.  Paper chromatography To set up paper chromatography: draw a pencil line near the bottom of the paper add spots of colourings to the line put the paper into a chromatography tank with a solvent at the bottom and below the line. The soluble dyes are carried up the chromatography paper. Different dyes travel upwards at different rates.  Chemistry - elements, mixtures and compounds Substances can be (1) pure elements, (2) pure compounds or (3) mixtures of elements or compounds. Atoms All substances are made of atoms. An individual atom is too small to see, so everything around you contains very many atoms. If an element is divided into smaller and smaller pieces, an atom is the smallest part that can exist. Elements An element is a substance that is made of only one sort of atom. Oxygen is an element because it contains only oxygen atoms. There are about 100 different elements. Atoms of each element are given a chemical symbol. Every symbol starts with a capital letter, often followed by a lower case letter. For example, N represents a nitrogen atom, but Na represents a sodium atom. •The elements are shown in the periodic table. Compounds Compounds form when atoms of different elements are combined during a chemical reaction. Compounds can separate back into elements only through chemical reactions. The different elements combined in a compound are in fixed proportions, shown in the chemical formula. Example H20 The formula of every water molecule is H20 because in each molecule two hydrogen atoms are chemically combined with one oxygen atom. Mixtures A mixture consists of two or more elements and/or compounds. These components are not chemically bonded. The chemical properties of each component in the mixture remain unchanged. Worked example Some iron powder and some sulfur powder are mixed, and tested with a magnet. The mixture is heated and cooled. The product is then tested with the magnet. Explain why the magnet attracts some of the mixture before heating, but none of the product after heating. (2 marks) The magnet attracts the iron, which is magnetic. When the mixture is heated iron sulfide is formed, which is non-magnetic. In the mixture, iron retains its magnetic property,  |
RSS Feed
