and their movement including:
i dilution of coloured solutions
ii diffusion experiments
Experiment 1: Dilution
Add a coloured substance to a solvent, e.g. food dye to a solvent, e.g. water. Over time, the intensity of the colour will decrease as it spreads throughout the liquid and dilutes- the concentration of the food dye decreases. Dilution increases at higher temperatures due to increased kinetic energy of particles, so applying heat decreases the time it takes for the dye to become fully diluted.
Experiment 2: Diffusion
Using a glass tube, place cotton wool soaked in ammonia solution at one side of the tube, and cotton wool soaked in hydrochloric acid at the other end. Both ends should be sealed with bungs. After a few minutes, a white ring of ammonium chloride will form in the tube, closer to the hydrochloric acid end. This tells us that the particles have diffused away from their respective ends to collide and react. The ammonia particles are smaller than the hydrogen chloride particles, and therefore move faster which is why the ring forms closer to the hydrochloric acid.
Experiment 3: Diffusion
Place liquid bromine at the bottom of a glass jar, and another empty jar upside down on top of it. As the bromine begins to evaporate, you can observe the volume of liquid decreasing, and the gas in the jar changing colour as the darker-coloured bromine particles diffuse upwards.
1.5 understand the terms atom and molecule
An atom is the smallest particle of an element that can exist. It has balanced charge (the same number of positively charged protons and negatively charged electrons) and a mass relative to carbon (12), made up of the neutrons and protons, each with mass of 1.
A molecule is a group of atoms bonded together, the smallest unit of a chemical compound that can take part in a chemical reaction, e.g. H2O
1.6 understand the differences between elements, compounds and mixtures
An element is one type of atom found on the periodic table, sometimes found bonded to another atom of the same element, e.g. O2, but it is always pure.
A compound is two or more elements chemically bonded in fixed proportions to form a new molecule, e.g. CO2
A mixture is two or more compounds or elements that are not chemically bonded, e.g. air, or a metal alloy such as steel.
1.7 describe experimental techniques for the separation of mixtures, including
simple distillation, fractional distillation, filtration, crystallisation and paper
chromatography
Solutions can be separated using a number of different techniques based on the physical differences of the substance (e.g. soluble and insoluble compounds). These include distillation, decanting, sieving, filtration, chromatography, crystallisation and magnetic separation.
Simple distillation: A liquid is boiled in a distilling flask, and the vapor travels through a tube, where thermal energy is passed to the cool water outside it and the vapor condenses into a liquid. The purified liquid then trickles out into a beaker. This generally is used to separate two liquids in a mixture, with different boiling points (e.g. ethanol and water)
Fractional distillation: The apparatus is similar to simple distillation, but the addition of a fractionating column filled with glass or plastic beads that allows better separation between the fractions due to condensation and re-evaporation occurring on the surface of each bead (distilling the mixture repeatedly). This allows the separation of more complex mixtures of liquids.
More on distillation
Filtration: A mixture is passed through a funnel with filter paper in it. The insoluble solid is left on the filter funnel, while the solution passes through the paper as the particles are small enough to fit through the gaps in the paper. This is used to separated a solution and insoluble particles.
Evaporation: A solution is heated to evaporate the liquid (e.g. water) leaving behind a solid that was previously dissolved. This is used to retrieve soluble compounds from solutions.
Crystallisation: A solution (e.g. aqueous sodium chloride) is heated to its boiling point, allowing the liquid, in this case the water, to partially evaporate. The solution is then left over a period of time, often several days to allow further water to evaporate at a lower temperature, causing crystals to form.
Chromatography: Different pigments in a mixture can be separated using chromatography. Dots of dye are placed at regularly spaced intervals at the bottom of a piece of chromatography paper, and the paper is placed in a beaker of solvent (the solvent should not reach the dye so it isn't dissolved and dissipated into the solution). The solvent will travel up the paper, and dissolve the pigments. Heavier pigments will be carried a shorter distance than pigments with smaller particles.
1.8 explain how information from chromatograms can be used to identify the
composition of a mixture.
Using a chromatogram, the composition of a mixture of dyes can be determined through comparison of different pigments. Different pigments are more or less water-soluble, and have different particle sizes, resulting in the pigments travelling a certain distance up the paper. By comparing a mixture of pigments, and marking the distance they travel, then comparing with individual pigments and the distance they travel, the composition can be determined. Pigments of the same kind will travel the same or similar distances.
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