P6 Chemistry Interesting Q/A

Note – This has been submitted by someone from o-levels, but most of it still applies to IGCSE

PAPER CHROMATOGRAPHY (ASCENDING)

Qn) Why is paper chromatography carried out in an enclosed environment? 

Ans) It serves to prevent the loss of the solvent from the tube by evaporation. 

Qn) Why is the starting line drawn with a pencil and not in ink? 

Ans) Graphite is insoluble but ink is soluble in ethanol. Hence, the starting line is drawn with pencil and not with ink so that the starting line material will not ascend the paper and interfere with the results of the paper chromatography. 

Qn) Why must the ink spot on the starting line be above the solvent line? 

Ans) When the dye is above the solvent, it will dissolve only when the ascending solvent reaches it and as a result, it will be carried upwards along the paper instead of spreading in all directions had it been submerged in the solvent. 

Qn) How is the distance travelled by the various dyes related to the solubility of the dyes in the solvent? 

Ans) The more soluble a dye is, the more readily it will dissolve in the solvent and will travel farther as it is being carried by the solvent being absorbed in the paper.

Qn) What would happen if a big spot of ink is used instead? 

Ans) Big ink spots tend to be separated into overlapping spots of dyes instead of being distinctly separated. 

PAPER CHROMATOGRAPHY (RADIAL) 

Qn) Compare the separation of dyes by the two types of paper chromatography 

Ans) In the ascending type, it is possible to have two separate paper chromatography occuring at the same time with the same solvent, thus the results can be compared with one another. However, radial chromatography can separate only one at a time. 

The radial type gives a faster separation because unlike the ascending paper chromatography, the dyes are not subjected to gravitation forces and thus the movement of dyes is faster. 

CRYSTALLIZATION 

Qn) Why is the filtrate heated gently? 

Ans) – to prevent overheating because then crystals won’t be obtained if it overheats as the filtrate will decompose to form simpler substances. 

• To prevent the filtrate from boiling and sputtering out of the evaporating dish. 

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Qn) How would we know if a solution is saturated? 

Ans) If a glass rod is dipped into the solution, a drop of the solution is allowed to cool on the rod and fine crystals are formed means that the solution is saturated. 

Separating smth that sublimes from smth that does not 

Qn) What is the use of the inverted filter funnel? What other alternatives could be used to replace the inverted filter funnel? (ammonium chloride and sand) 

Ans) It is used to provide a cold surface for the vapours (ammonia and hydrogen chloride) to change back to the solid state. A round bottom flask or an evaporating dish filled with extremely cold water or ice can also be used. 

Qn) Can a salt that sublimes be separated from sodium chloride by heating? 

Ans) Yes. Sodium chloride has a high melting point and can withstand high temperatures without decomposing. 

Qn) Iodine is also separated using sublimation from a mixture but it is carried out in the fume cupboard. Why? 

Ans) Iodine fumes are poisonous. It is toxic and irritant to the eyes. 

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Context : The time taken for the solute to dissolve in the solvent varies according to the temperature of the solvent. 

Qn) Identify one key source of experimental error and explain how it affects the experimental results. 

Ans) Heat loss to the surroundings is the experimental error and this causes the time taken to be longer than expected. 

Qn) Describe a precaution that you would take to ensure that the volume of the water/any liquid measured by the measuring cylinder is accurate? 

Ans) Prevent parallax error by ensuring that the reading taken is at eye level. 

Qn) Why is it necessary to stir the water while it is being heated to achieve the desired temperature? 

Ans) To ensure even distribution of thermal energy throughout the water so the temperature is uniform throughout. 

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• When doing a cation test, write down the observation upon adding a little and upon adding excess 

Qn) What is the purpose of using distilled water for any experiment? 

Ans) To ensure that no ions are introduced into the solution. 

Qn) Why are cation tests not performed using the solid form of the salt? 

Ans) To prevent precipitation of insoluble hydroxides on the solid, which will cause the reaction to stop. 

Qn) Why is aqueous NaOH/NH3 added little by little? 

Ans) Some metal hydroxides (ppt) are soluble in excess NaOH/NH3 hence we won’t know its solubility in a little amount if too much is added at once. No observation to obtain a conclusion of the identity can be made. 

Qn) Why are they later added in excess? 

Ans) To test the solubility of the ppt. 

Qn) Why do we shake a mixture? 

Ans) To ensure that they are indeed immiscible. 

• If a reaction mixture is warmed then it is probably to test for the gas evolved. 

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Titration 

• Burette reading (to 2dp) 
• 2 drops of indicator are enough 
• The 2 best burette readings must be within 0.2cm^3 of one another. 
• All readings should be taken at eye level to eliminate parallax error. 
• Rinse burette, filter funnel and pipette with tap water and then rinse the burette and pipette with the specified solutions. 
• Rinse the conical flask with only distilled water. 
• Ensure that there are no air bubbles in the pipette. 
• When you transfer the solution in the pipette to the conical flask, there will always be a little amount of liquid left at the top of the pipette. This has already been accounted for in the calibration of the pipette. DO NOT BLOW INTO THE PIPETTE TO EXPEL THIS SMALL AMOUNT OF LIQUID. 
• Heating the acid or the alkali does not change the volume of anything or the concentration but simply makes the speed of reaction faster. 

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Some general notes 

Calculations : all to 3 sf except average volume of the burette solution used. 

Experimental errors : do not include errors that can be avoided / eliminated by taking precautions (eg. parallax errors, inconsistent swirling, etc) 

Possible errors : presence of acidic impurities in the unknown acid or the presence of alkaline impurities in the unknown alkali. ( this can cause the volume of solution needed to be wrong ) 

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Planning Questions 

1. Identify the I.V. , D.V. and CV 
2. Outline an experimental procedure to investigate the qn/problem. 
3. Describe how the data should be used in order to reach a conclusion. 
4. Identify the risks and state the precautions that should be taken to keep risks to a minimum. 
5. You can support your answer with a diagram if the question requests. 

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QA OBSERVATIONS

• State the colour and odour of the gas formed, then the observation for the confirmation test. 
• State the initial and final colour of the solution. 
• When a solid dissolves/disappears, record the colour of the solution formed.
• State the colour of the ppt. 
• If there is a difference in the colour of the ppt and the solution then state both. 

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When using a bunsen burner,

Always use a non luminous flame

• When lighting the bunsen burner, ensure that the airhole is closed. 
• Eventually, make sure that the airhole is partially open. 

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Graphs 

• Scaling must be easy to interpret. 
• Graph must cover more than half of the paper.
• Remember to write a proper title. 
• Label both the axis’
• Independent variable is usually on the x axis 
• Dependent variable is usually on the y axis 
• A straight line indicates a direct relationship between the two variables. 
• A curve indicates a changing relationship between the two variables. 

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– sources of error can be human reaction time, parallax error and experimental errors. 

Some common experimental errors: 

• Replace a less precise measuring instrument with a more precise one to improve the accuracy of your results. 
• If a solution is poured into a beaker then it has a large surface area which will increase the rate of evaporation so the reading of the volume will be lesser than expected. 
• When doing evaporation, do it in an evaporating dish to provide a larger surface area so that the rate of the evaporation will be faster. 
• Heat loss to the surroundings is an exothermic reaction which will cause the highest temperature reached of the reaction to be lower than expected. One improvement is to nest the styrofoam cup into a beaker so that the beaker can provide thermal insulation.
• Strong heating on the evaporating dish may cause the salt formed to sputter, thus the mass recorded will be lower than expected. 
• In acid-alkali reactions, the acidic/alkali impurities present will affect the final burette reading. 

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Imagine a titration using HCl. What happens if we used sulfuric acid and ethanoic acid instead? 

1. H2SO4 : It is a dibasic acid and thus has double the concentration of H+ ions as compared to hydrochloric acid. Since the number of mols of alkali is the same, the same number of mols of H+ ions will be needed, thus the volume of acid required is halved. 
2. CH3COOH : It is a monobasic acid and thus has the same concentration of H+ ions as compared to HCl. The only difference is that the dissociation of the H+ ions from the weak ethanoic acid will be slower. Since the number of mols of alkali is the same, the same number of mols of H+ ions would be needed and thus, the volume of acid needed is the same. 

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Qn) Reason of plugging the mouth of a flask with cotton wool: 

Ans) Prevent any unintended loss of mass due to the loss of ____ (soln) while letting ____ (gas) escape to the surroundings. 

Qn) Why is the top of the meniscus read instead of the bottom when dark liquids are used ? 

Ans) The dark _____ (colour) is too dark for the lower meniscus of the solution to be read off clearly. To minimize/reduce inaccuracy, the top of the meniscus is read off instead. 

Qn) How do we increase the accuracy of the temperature recorded? 

Ans) A temperature sensor can be connected to a data logger to measure the temperature.