Tips on how to solve last 10 questions on chemistry Paper 1 (MCQ):
0. (computer scientists (be it prospective) always count from zero..)Read the question completely, no matter how long it is.
1. Try to find the chapter the question might have come from. Or at least know if it is from physical, organic or inorganic.
2. Consider each statement using chemistry principles and logic.
3. Eliminate choices smartly.
4. Reach to the final answer.
Example: Maj/June 2009, Q 35 : The rock dolomite is a double carbonate of magnesium and calcium , CaCO3 . MgCO3
When heated strongly, a product called calcined dolomite is is formed which is used to line some furnaces for the production of metals.
Why calcined dolomite is used for this purpose?
1. It is a refractory material.
2. It will absorb acidic impurities in metallurgical processes
3. It will reduce metallic oxides to metals
There should be a preliminary thought (PT) before considering the statements.
PT: The question is from Group II elements and involves heating of carbonate. So, it has to deal with thermal decomposition of carbonates and the product formed after decomposition: CaCO3 >>>heat>>> CaO + CO2
MGCO3 >>> hear >> MgO + CO2
Most probably, the calcinated dolomite will contain the mixture of MGO and CaO as CO2 will escape being gas.
Statement 1: This can be tackled in two ways
Simple case: (Useful for AS students) : The question itself has the answer. Read the line '....which is used to line some furnaces....'. You can be sure that statement 1 is correct unless your vocabulary prevents you from understanding the word 'refractory'
Case II: (For A2 students only). You have read that MgO has high exothermic lattice enthalphy, which means that large heat input is required to break its strong lattice. Hence, it is thermally stable or it has large melting point. So, it can relatively withstand high temperature and is ideal for its use in refractory lining of furnaces.
Now, we know st. 1 is correct. We now can eliminate choice C
Statement 2: CaO and MgO both are basic oxides so will react to acidic impurities. St. 2 correct so eliminate choice D.
Statement 3: (This is my personal logic, I mean 'guess'. If anyone can come up with better explanation, you are welcome)
The cations present are Ca2+ and Mg2+. To reduce others these cations must get oxidised. I have not seen cations like Ca3+ or Ca4+, so I assume that these cations cannot be oxidised. After all, they are not transistion elements to show variable oxidation states. Statement 3, by my guess, is incorrect.
So I think the correct answer is B.
My logic for statement 3 worked. The mark scheme says B is the correct answer.
This long explanation may seem overwhelming. But you are not to write this long explanation, you simply reason inside your brain and use space for rough working. The last effort is to shade the correct choice which is effortless and the entire process takes around one minute or even less.
0. (computer scientists (be it prospective) always count from zero..)Read the question completely, no matter how long it is.
1. Try to find the chapter the question might have come from. Or at least know if it is from physical, organic or inorganic.
2. Consider each statement using chemistry principles and logic.
3. Eliminate choices smartly.
4. Reach to the final answer.
Example: Maj/June 2009, Q 35 : The rock dolomite is a double carbonate of magnesium and calcium , CaCO3 . MgCO3
When heated strongly, a product called calcined dolomite is is formed which is used to line some furnaces for the production of metals.
Why calcined dolomite is used for this purpose?
1. It is a refractory material.
2. It will absorb acidic impurities in metallurgical processes
3. It will reduce metallic oxides to metals
There should be a preliminary thought (PT) before considering the statements.
PT: The question is from Group II elements and involves heating of carbonate. So, it has to deal with thermal decomposition of carbonates and the product formed after decomposition: CaCO3 >>>heat>>> CaO + CO2
MGCO3 >>> hear >> MgO + CO2
Most probably, the calcinated dolomite will contain the mixture of MGO and CaO as CO2 will escape being gas.
Statement 1: This can be tackled in two ways
Simple case: (Useful for AS students) : The question itself has the answer. Read the line '....which is used to line some furnaces....'. You can be sure that statement 1 is correct unless your vocabulary prevents you from understanding the word 'refractory'
Case II: (For A2 students only). You have read that MgO has high exothermic lattice enthalphy, which means that large heat input is required to break its strong lattice. Hence, it is thermally stable or it has large melting point. So, it can relatively withstand high temperature and is ideal for its use in refractory lining of furnaces.
Now, we know st. 1 is correct. We now can eliminate choice C
Statement 2: CaO and MgO both are basic oxides so will react to acidic impurities. St. 2 correct so eliminate choice D.
Statement 3: (This is my personal logic, I mean 'guess'. If anyone can come up with better explanation, you are welcome)
The cations present are Ca2+ and Mg2+. To reduce others these cations must get oxidised. I have not seen cations like Ca3+ or Ca4+, so I assume that these cations cannot be oxidised. After all, they are not transistion elements to show variable oxidation states. Statement 3, by my guess, is incorrect.
So I think the correct answer is B.
My logic for statement 3 worked. The mark scheme says B is the correct answer.
This long explanation may seem overwhelming. But you are not to write this long explanation, you simply reason inside your brain and use space for rough working. The last effort is to shade the correct choice which is effortless and the entire process takes around one minute or even less.
