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).
But the change in color is due to change of cuso4 from anhydrous state to hydrated state. The oxidation number of cu2+ is unchanged. so (3) should be incorrect!!!
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?
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.
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.
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.
27 留言
For question 1 why the answer is B? It seems that simple distillation should be the best option
回覆刪除there are impurities in sea water
刪除大澳...
刪除After simple distillation, the distillate is distilled water while the solids left behind are a mixture of NaCl, MgCl2, etc.
刪除有補習名師話第一條係A既
刪除Crystalization 都有impurity
刪除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.
刪除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).
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.
回覆刪除No because I remember the question stated “until in excess”
刪除Tetrahydroxozincate ion.....i think it's spelt like this...lol
刪除第六題係B 因為in excess
刪除ZnSO4 同PbNO3 會redissolve
而Iron(III) 係yellow/brown precipitate所以唔係ACD
Oh I see. Thanks for your help. 0000
回覆刪除Q32 (3) is correct?
回覆刪除copper 2 sulphate powder is white, it turns blue when dissolved in water due to cu2+ ions....this is also a test for water...
刪除transition metal ions are colored, so it is correct.
But the change in color is due to change of cuso4 from anhydrous state to hydrated state. The oxidation number of cu2+ is unchanged. so (3) should be incorrect!!!
刪除Q17...will the seperation actually increase? it is still liquid, still packed irregularly but closely, wont that only happen for gases?
回覆刪除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?
Q17 Have you heard of thermal expansion? 冷縮熱脹
回覆刪除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.
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.
刪除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.
刪除anyone have 2018 dse chem paper? I wanna have it and check with you
回覆刪除作者已經移除這則留言。
回覆刪除今年算簡單
回覆刪除有冇人可以upload份題目？
回覆刪除can anyone upload the paper??
回覆刪除Very easy this year
回覆刪除Predict 92% or above for 5**
do you have paper?
刪除幾成有5
回覆刪除