Chemistry/Identifying and Making Mixtures Marking Scheme

Science 8 Experiment MARKING SCHEME for IDENTIFYING MIXTURES VERSUS PURE SUBSTANCES AND MAKING MIXTURES

Maximum = 12

OBSERVATIONS: (4; –1/2 per incomplete or poor observation) (b)	When salt water is filtered through a filter paper all the liquid comes through the paper and nothing is left behind on the paper.
 * 1) The liquid evaporates, leaving some blue crystals behind. Tube A: 2 mL of saturated copper sulphate solution
 * 2) Small crystals start to form when the liquid evaporates. Tube B:   2 mL of saturated sodium chloride solution
 * 3) The liquid evaporates and nothing is left behind. Tube C:   2 mL of ethanol
 * 4) The test tube contains a white solid. When water is added, some of the solid dissolves and some does not dissolve.  Tube D:  1 mL of a 1:10 mixture of copper(II) sulphide and sodium chloride, finely powdered and thoroughly mixed together.
 * 5) The test tube contains blue crystals. When water is added everything dissolves to give a blue liquid.  Tube E:  1 mL of powdered cupric sulphate
 * 6) The test tube contains a clear slightly yellow liquid. When mixed with water a jelly-like mixture is created with one liquid floating on top of another.  Tube F:  2 mL of salad oil
 * 7) The test tube contains a clear and colourless liquid. When mixed with water a clear, colourless liquid results.  Tube G:  2 mL of vinegar
 * 8) The test tube contains a clear, thick liquid. When shaken with water a clear liquid is eventually formed.  Tube H:  1 mL of glycerine
 * 9) The test tube contains a white solid. When water is added a suspension is formed.  Tube I:  0.2 g of starch
 * 10) Test tube J contains a clear, colourless solution. Test tube K contains a clear, colourless solution. When the contents of the tubes are mixed, the solution is initially clear and then becomes more and more cloudy. Tube J:  5 mL of 0.2 M sodium thiosulphate, Na2S2O3•5H2O. Tube K:  10 drops of 2 M hydrochloric acid
 * 11) (a)	When a sand–water mixture is filtered, the water comes through the filter paper and the 	sand remains behind in the filter.

DISCUSSION: '''1.	Look at the observations you made when the liquids in test tubes A, B and C are allowed to evaporate. Which of these test tubes, if any, contain solutions? Which, if any, contain mechanical mixtures? Which, if any, contain a suspension? Which, if any, appear to be pure substances?''' (1.5) Tubes A and B contain a solution; C contains a pure substance.

'''2.	According to your observations, does test tube D contain a pure substance or a mixture? Does test tube E contain a pure substance or a mixture?''' (1) Tube D contains a mixture and tube E contains a pure substance.

'''3.	Look at the observations you made when the liquids in test tubes F, G, H and I are mixed with water. After water is added, which of the mixtures, if any, are solutions? Which of the mixtures, if any, are mechanical mixtures? Which of the mixtures, if any, are suspensions?''' (2) Tube F contained a mechanical mixture; tubes G and H contained solutions. Tube I contained a suspension.

'''4.	Look at the observations you made in Procedure step 10. A chemical reaction occurred in the test tube. Did the reaction produce a solution, a mechanical mixture or a suspension? Explain why you believe your answer is correct.''' (1) The reaction produced a suspension because the mixture was cloudy.

'''5.	Look at the observations you made in procedure step 11. (a)	Which of the two mixtures is a mechanical mixture and which is a solution? (b)	Is it possible to separate a mechanical mixture by filtration? Explain why you believe your answer is correct. (c)	Is it possible to separate a solution by filtration? Explain why you believe your answer is correct. '''

(1) a)	The sand–water mixture is a mechanical mixture and the salt water is a solution. 	(1) b)	A mechanical mixture can be separated by filtration. The sand was left behind on the filter paper and the water went through the paper. (1/2) c)	A solution cannot be separated by filtration. Nothing was left behind on the filter paper.