For those local students that did not do well for the O Level Chemistry Paper 2 on Oct 20th, do not be discouraged because there is one more chance for you to UP your Chemistry Grades.
You still have your Chemistry Paper 1 (Multiple Choice Questions) Exams in 19 days time. There is more than enough time for your to re-learn and re-fine your Chemistry Concepts and earn high marks for it.
There are 40 marks for you to bag in Chemistry Paper 1 and weightage is as high as 30%!
But there is one problem i usually see. Students tends to let off their guards and relax before the Paper 1 because:
1. Chemistry Paper 1 is the last paper of GCE O Level Exams and students already in “holiday mood”
2. Many students feel that?Paper 1 (MCQs) is very easy .i.e relax mood….just go there select choices
They are certainly wrong about this. MCQs is the one that truly tests you on your concepts and applications.
In Paper 2 (Written Theory), questions are usually phrased and linked from one part to another, to help you to get some marks here and there.
But in Paper 1 (MCQs), you will score only either when you know the concepts really well and know how to apply, or when you are lucky. For me, i don’t believe in luck – i prefer to control my own destiny.
Now, that is the reason you see me starting this Mini Series on Tough MCQs.
And for those that wish to score an A1 for overall Chemistry Exams, i can assure you that MCQs will ultimately determine whether you get an A1 or A2! Trust me on that cause i have been through that, just like hundreds of students that i have coached before.
Below are sets of Exam-based MCQs for you to try:
Question 1:
When crystals of copper(II) sulphate are heated, the color changes from blue to white. This is caused by:
A. loss of water only
B. loss of water and sulphur trioxide
C. loss of water, sulphur dioxide and oxygen
D. reaction with carbon dioxide in the air
E. reaction with water vapour in the air
Question 2:
Which of the following salts can be crystallized from aqueous solution?
A. barium sulphate
B. calcium carbonate
C. potassium nitrate
D. silver chloride
E. zinc carbonate
Question 3:
Which one of the following hydroxides does not give a good yield of a salt with dilute sulphuric acid?
A. Iron(II) hydroxide
B. Magnesium hydroxide
C. Zinc hydroxide
D. Calcium hydroxide
E. Copper(II) hydroxide
PS: Do try it out and leave your comments below. Suggested answers will come in with reasonings.
Hi Hecate.
I reckon you are commenting on Question 1 of the Multiple Choice Questions.
Yes in a way, it is due to the loss of water but we normally do not call it “dehydration”.
For ionic salts which are soluble in water, there will be something known as the “Water of Crystallisation”.
Many salts combine with water molecules to form CRYSTALS. These water molecules are known formally as “Water of Crystallisation”. The water of crystallisation can be removed by heating Hydrated Salts. When heated, the water of crystallisation is given off and the resulting salt is known as Anhydrous Salts. Such anhydrous salts are often seen as powders.
The hydrated salt can be obtained from the anhydrous salts by adding water.
Hope you find the above discussion useful to your Chemistry learning.
Sean Chua
Dehydration…is that right?
Q1. A, because the colour of copper sulfate changes from blue to white due to the removal of the water of crystallization.
Q2. C, because only soluble salts can be crystallized which is potassium nitrate in this case.
Q3. D, because an insoluble salt i.e calcium sulfate is formed by the reaction of calcium hydroxide and sulfuric acid. Calcium sulphate then covers the alkali and the reaction does not proceed smoothly.
my answer are A,C and D
Well Done Sadiqin.
Your answers are all correct.
Question 1:
Colour of copper(II) sulphate crystals are indeed due to the Water of Crystallization.
Question 2:
Based on the Table of Solubility of Salts (which is required to memorise and know well for O Level Chemistry), Potassium Nitrate is the only salt in the list to be soluble in water, and can be crystallized from its aqueous solutions.
Question 3:
Many students got this question wrong in my chemistry workshops and bootcamps when i tested them.
Many chose Copper Hydroxide as the answer which is incorrect. Just because Copper is one of the least reactive metals in the Metal & Reactivity Series, they thought Copper(II) Hydroxide will be unreactive. This is NOT true. Copper(II) Hydroxide does has the properties just like any other alkalis.
The clue for this question comes from “does not give a good yield of the salt”.
It hints that the salt will not form properly due to some reasons. Based on Table of Solubility of Salts, Calcium Sulphate that is formed between Sulphuric Acid + Calcium Hydroxide is INSOLUBLE in Water. As such, it will form a layer around the calcium hydroxide and prevents reaction from continuing smoothly. The same does not happen for other salts given, which are soluble in water.
Cheers
Sean
My answers are A, C and D respectively.
The first question is quite alright. Hopefully everyone knows that the water of crystallisation (or simply water molecules) is what gives the copper(II) sulphate it’s colour.
Potassium nitrate is the only one which is soluble in water. They say aqueous solution, hence I gather they’re asking us which one of those salts are soluble in water.
Question three requires us to spot the odd one out. In this case, it’s calcium hydroxide. When Calcium hydroxide reacts with sulphuric acid, it produces a calcium sulphate, which is insoluble, and hence will form a layer around the calcium hydroxide and stop the reaction from continuing. The same doesn’t happen for the other salts given in the question.