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2018-04-23

【CHEM】2018 DSE Paper 1A(MC) & Paper 2 suggested answer

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Paper 1A
1-5:      BCDDC
6-10:    BBCAA
11-15:  DBBCC
16-20:  ADADA
21-25:  BACCD
26-30:  BABBD
31-36:  BDACA   C

Paper 2 
PDF



Explanation
Note: 


continue...

26 則留言:

  1. For question 1 why the answer is B? It seems that simple distillation should be the best option

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    1. there are impurities in sea water

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    2. After simple distillation, the distillate is distilled water while the solids left behind are a mixture of NaCl, MgCl2, etc.

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    3. 有補習名師話第一條係A既

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    4. Crystalization 都有impurity

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    5. In water, the concentration of NaCl is higher than other impurities such as MgCl2. So, during crystallization, the solution will be saturated in terms of NaCl, so NaCl crystallizes out as solid. The other impurities, having lower concentrations, are not yet saturated and will not come out as solid crystals. Unless too much water is evaporated that the impurities get saturated, the crystals formed will be pure.
      For A, the products from electrolysis are H2 (g), Cl2 (g) and NaOH (aq). No NaCl can be obtained directly (unless you react Cl2 (g) with NaOH (aq), which will also give NaOCl to contaminate the NaCl).

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  2. For question 6, can the answer be A since Ca(OH)2 like limewater is a colorless solution and also, zinc will form white ppt before it redissolves.

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    1. No because I remember the question stated “until in excess”

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    2. Tetrahydroxozincate ion.....i think it's spelt like this...lol

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    3. 第六題係B 因為in excess
      ZnSO4 同PbNO3 會redissolve
      而Iron(III) 係yellow/brown precipitate所以唔係ACD

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  3. Oh I see. Thanks for your help. 0000

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    1. copper 2 sulphate powder is white, it turns blue when dissolved in water due to cu2+ ions....this is also a test for water...

      transition metal ions are colored, so it is correct.

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  5. Q17...will the seperation actually increase? it is still liquid, still packed irregularly but closely, wont that only happen for gases?

    Q24...isnt nh3 a weak acid, though the equation ratio is 1:1, but the [oh-] would be lower than that in koh which ionises completely? so shouldn't more nh3 be needed?

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  6. Q17 Have you heard of thermal expansion? 冷縮熱脹
    Q24 NH3 <> NH4+ + OH-
    When OH- reacts with acid, reaction will shift right to form more OH-. Eventually all the NH3 will ionize as OH- is constantly removed. So, the number of moles of OH- supplied by NH3 and KOH are the same.

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    1. Q17 There is only increase in temperature and have no change of state of H2O. Therefore molecular potential energy between H2O molecules remains unchanged which means no change in average distance. So (3) is incorrect and the answer should be B.

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    2. Thanks for the answer, especially to 17... I think the supplier is right. Although in physics we are taught the PE remains unchanged, but that isn't necessarily the case. I just checked the actual graph on google, look it up. Thermal expansion is the answer, too bad it slipped from my mind during the exam.

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  7. anyone have 2018 dse chem paper? I wanna have it and check with you

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  8. 作者已經移除這則留言。

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  9. 有冇人可以upload份題目?

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  10. can anyone upload the paper??

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  11. Very easy this year
    Predict 92% or above for 5**

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