CHEMISTRY AQA GCSE

• Elements:

  • Around 100 different elements shown in the periodic table
  • Each symbol in the periodic table represents a different element
  • Definition: In an element, all atoms are the same
  • Example: Magnesium and sulfur are two different elements
  • Every element has a symbol starting with a capital letter
• Compound:

  • Formed by chemically combining atoms of different elements
  • Definition: Compounds contain two or more different elements chemically combined in fixed proportions
  • Example: Magnesium sulfide has a fixed proportion of magnesium and sulfur atoms
  • Compounds usually have different properties than the elements they are made from
• Mixture:

  • Contains different elements or compounds not chemically combined
  • Can be separated using physical separation techniques like filtration, distillation, crystallization, or chromatography
• Molecule:

  • Any elements chemically combined, even if they are the same element
  • Examples of molecules: methane, water, ammonia
  • Molecules can be compounds if they contain different elements chemically combined
  • Elements like chlorine and oxygen can form molecules but are not compounds
• Chemical formula tells us the elements in a molecule and the numbers of atoms of each element
• In a specific compound, the number of atoms of each element is fixed
• Changing the number of atoms of each element results in a different compound
• Physical separation techniques can only be used to separate mixtures, not elements in a compound
• Filtration is used to separate an insoluble solid from a liquid
• Insoluble means the solid will not dissolve in the liquid
• Filtration uses a filter funnel and filter paper to separate the solid from the liquid
• Solid material is trapped by the filter paper, while the liquid passes through as filtrate
• Crystallization is used to separate a soluble solid from a liquid
• Sodium chloride is soluble in water, forming a sodium chloride solution
• Crystals of solid sodium chloride form when the water evaporates from the solution
• Heating the solution can speed up crystallization, but some chemicals may break down if heated
• what is simple distillation?

  when you want to separate the liquid and solid- but keep the liquid. It involves evaporating the liquid by heating and then condensing it (By using a condenser).
• Cold water from the tap continuously runs through the condenser
• You start simple distillation by heating the solution
• fractional distillation is used to separate a mixture of liquids
• fractional distillation can only be done if the boiling points of the different liquids are different to each other- as that is what it relies on
• ONLY in FRACTIONAL DISTILLATION do you have a fractionating column which is a column which causes the liquid (once it has evaporated) to condense and causes it to accumulate- increasing the amount of the lower boiling point liquid
• When the thermometer reaches a constant temperature (for the first time), it means that the chemical with the lowest boiling point is coming through the condenser, and therefore through the beaker. And the second time the thermometer is constant, its then the chemical with the higher boiling point
• in fractional distillation, if the two liquids have very similar boiling points, then they will be very hard to separate
• to make the liquids very pure, you may have to complete several rounds of fractional distillation
• Paper chromatography is a physical process that allows the separation of substances in a mixture based on their different solubilities
• The paper used in chromatography is called the stationary phase because it does not move
• The solvent used in chromatography is called the mobile phase because it moves
• A pure compound will produce a single spot in all solvents, whereas compounds in a mixture may separate into different spots depending on the solvent
• Different substances have different solubilities, with more soluble substances being more attracted to the mobile phase (solvent) than less soluble substances
• More soluble substances travel further than less soluble substances in paper chromatography
• Drawing the starting line in pencil is important to prevent the ink from dissolving in the solvent and moving up the paper
• Plum Pudding Model of Atomic Structure:

  • An atom is a ball of positive charge with negative electrons embedded in it
  • Proposed as a new model for the structure of atoms in response to the discovery of electrons in 1897
• Alpha Scattering Experiment:

  • Scientists used a piece of gold foil for the experiment
  • Fired alpha particles (positively charged) at the gold foil
  • Most alpha particles passed straight through the gold foil, indicating that atoms are mainly empty space
  • Some alpha particles were deflected, suggesting the center of the atom has a positive charge
  • Some alpha particles bounced straight back, indicating the center of an atom contains a great deal of mass
  • Led to the replacement of the Plum Pudding Model with the Nuclear Model
• Nuclear Model of Atomic Structure:

  • Most of the atom is empty space
  • The center of the atom contains a tiny positive nucleus with most of the mass
  • Negative electrons are found around the edge of the atom
• The nuclear model of atomic structure replaced the Plum Pudding model after the results of the alpha scattering experiment
• In the nuclear model, most of an atom is empty space, with a positive nucleus in the center containing most of the mass and negative electrons around the edge
• Electrons orbit the nucleus at specific distances called energy levels or shells, proposed by Neils B based on calculations and experiments
• The nucleus contains protons, which are tiny positive particles determining the positive charge in the nucleus
• The nucleus also contains neutrons, discovered by James Chadwick, which are neutral particles