SC2 - Methods of Separating and Purifying Substances Flashcards
SC2a
1) What is the composition of a pure substance like?
2) What is a mixture?
3) Explain why the temperature does not change as a pure substance melts.
1) The composition of a pure substance: consists only of one element or one compound. For example, pure gold only contains gold atoms.
2) A mixture contains elements and/or compounds that are not chemically joined together. You can use physical processes to separate mixtures. A mixture does not have a fixed composition.
3) A pure substance has the same composition in every part of it, so it’s physical properties are all the same. This means that all of a pure substance will melt at the same temperature.
SC2a
1) What is the melting point?
2) What is the melting point like in a pure substance?
3) What is the melting point like in a mixture?
1) The melting point is the temperature at which a solid becomes a liquid. This is an example of a physical property.
2) A pure substance has the same composition throughout, and therefore the physical properties are the same in every part of it. This causes the melting point to be one single temperature (eg. 12°C).
3) Mixtures do not have a fixed, sharp melting points; they melt over a range of temperatures (eg. 2°C - 8°C), as not all the parts melt and become liquid at the same time.
SC2a
1) What does the heating curve look like for a pure substance?
2) What does the heating curve look like for an impure substance?
3) What happens to its particles when a solid melts?
1) On a heating curve, the point at where a pure substance melts is a straight horizontal line, as it only has one melting point.
2) On a heating curve, the point at where a mixture melts is a diagonal line, as it has multiple melting points.
3) When a solid melts, its particles gain enough energy to overcome the weak forces of attraction between them. They move further away from one another and the solid becomes a liquid.
SC2b
1) What are filters used for?
2) What is a solution?
3) What is a solute, and what is a solvent?
4) What is a saturated solution?
1) Filters can be used to separate an insoluble solid from a liquid. They let smaller pieces or liquids through but trap bigger pieces or insoluble substances.
2) A solution is a mixture made from solutes (dissolved substances) in a liquid called the solvent.
3) A solute is the dissolved substance, and the solvent is the liquid that the solute is dissolved in.
4) A saturated solution is a solution that contains the maximum amount of dissolved solute at that temperature.
SC2b
1) How does the filtration practical work?
2) How does the crystallisation practical work?
1) A filter funnel is lined with filter paper that has fine holes in it. The solvent and solute(s) pass through the fine holes to form the filtrate. Bits of insoluble substances cannot fit through the holes and so leave a residue in the filter paper.
2) A Bunsen Burner is used to evaporate the solution, which is placed in an evaporating basin. The solvent will evaporate and leave the solute behind as crystals.
SC2c
1) What is chromatography?
2) What is paper chromatography?
3) What is the Rf value, and what can it be used to do?
1) Chromatography can be used to find out which coloured compounds a mixture contains.
2) Paper chromatography is a simple technique where a solvent moves along a strip of paper, it carries the different substances in the mixture at different speeds, so they are separated.
3) The Rf value is the distance the compound has risen divided by the distance the solvent as risen. It can be used to distinguish between pure and impure substances, and identify substances by their Rf values.
SC2c
1) How do you identify pure substances and mixtures on chromatograms?
2) How do you identify substances that are the same on chromatograms?
3) Explain how chromatography works
1) Pure substances will produce only one spot on the chromatogram. If the substance is a mixture, it will separate on the paper to show all the different components as separate spots.
2) Two substances are likely to be the same if: they produce the same number of spots, and these match in colour, and the spots travel the same distance up the paper.
3) A pencil line is drawn and spits of ink are placed on it. There is a container of solvent such as water. The paper is lowered into the solvent. The solvent travels up through the paper, taking some of the coloured substances with it. As the solvent continues to travel up the paper, the different coloured substances spread apart.
SC2c
1) What is the solvent called in chromatography?
2) What is the paper called in chromatography?
3) What is the paper with the separated inks on it called in chromatography?
1) The solvent is called the mobile phase.
2) The paper is the stationary phase.
3) The paper with the separated inks on it is called the chromatogram.
SC2b
1) What is a risk assessment?
2) What is a hazard?
3) How can risks be reduced during crystallisation?
1) In a risk assessment, the hazards of doing an experiment are identified.
2) A hazard is something that could cause harm. Then ways of reducing the risk(chance) of a hazard causing harm is considered.
3) During crystallisation, the risks from spitting can be reduced by wearing eye protection, removing the Bunsen burner before the solution is completely dry, and/or using steam to heat the basin gently.
SC2c
1) Name the parts of the filtration laboratory apparatus.
2) Name the parts of the crystallisation laboratory apparatus.
1) Filter paper, filter funnel, conical flask, suspension (the mixture), the residue, and the filtrate.
2) The filtrate/solution , evaporating basin, the gauze (thin metal put on a tripod to support the basin) , tripod and the Bunsen burner.
SC2d
1) What is distillation?
2) Describe how simple distillation works.
3) Name the parts of the simple distillation laboratory apparatus.
1) Distillation is the process of separating a liquid from a mixture by evaporating the liquid and then condensing it.
2) The apparatus used is called a still. Water is heated in the distillation flask, evaporates, and the water vapour cools in the condenser and drips into a conical flask or beaker. The solute is left behind in the distillation flask.
3) A thermometer, distillation flask, tripod, Bunsen Burner, condenser (a central tube surrounded by a jacket of cold water), the solution (eg. salty water) and distillate (eg. pure water), the conical flask or beaker containing the distillate, and anti-bumping granules (to make the liquid boil more smoothly — small bubbles of vapour form on the corners of the granules and reduce the risk of the liquid boiling over).
SC2d
1) What is fractional distillation?
2) Describe how fractional distillation works.
3) Name the parts of the fractional distillation laboratory apparatus.
1) Fractional distillation is a method of separating a mixture of liquids with different boiling points into individual components (fractions).
2) In fractional distillation, a column is fixed above the distillation flask. The hot vapour rises up to the column. At first the vapour condenses, but as the column gradually heats up, there will be a temperature gradient; it will be hottest at the bottom, and the temperature will drop as you go further up the column. The fraction with the lowest boiling point will reach the top of the column first, and the vapour will then pass into the condenser.
3) Distillation flask, thermometer, condenser, fractionating column, beaker.
SC2d - Core Practical
1) What is the aim of the distillation core practical?
2) What is the method for simple distillation?
1) Investigate the composition of inks using simple distillation.
2) Simple Distillation Method:
A) Set up your apparatus so that the ink is in a flask, and its vapours can be led away to be condensed.
B) Heat the flask of ink using a Bunsen Burner, making sure the ink simmers gently and does not boil over into the delivery tube.
C) Continue being heated until you have collected a few cm3 of distillate (distilled solvent).
D) Note the maximum temperature obtained.
SC2d - Core Practical
1) What are the issues/ key points of the distillation practical (state 2)?
2) What are the hazards and precautions for the distillation practical (state 1)?
1) The flask should not be heated too strongly as this could cause the water to boil over into the beaker collecting the distillate.
Also, the condenser should be horizontal, with water entering at the bottom and leaving at the top.
2) A hazard is the Bunsen Burner flame, or hot apparatus. The risk is that it could burn skin. The precautions are to make sure that hair and clothes are tucked in (so they don’t catch fire), and to allow the apparatus to cool before touching it.
SC2d - Core Practical
1) What is the aim of the investigating inks core practical?
2) What is the method for paper chromatography core practical?
1) Investigate the composition of inks using paper chromatography.
2) Paper Chromatography Method
A) Draw a pencil line on a piece of chromatography paper, about 2 cm from the bottom.
B) Add a small spot of ink to the pencil line, and add water to a container of a depth of about 1cm.
C) Place the paper into the container. Make sure the paper is supported so that it does not slump into the container when it becomes damp. Also, make sure that the water height does not go over the pencil line. Allow the water to travel through the paper.
D) Take the paper out before the water reaches the top. Immediately mark the position of the solvent front using a pencil, then leave the paper to dry.
E) Measure the distance travelled by the water from the pencil line, and the distances travelled by each coloured substance. Then, calculate the Rf value for each coloured substance.
