Chemistry Class 12 NCERT Solutions: Chapter 6 General Principles and Processes of Isolation of Elements Part 3

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Q: 11. What criterion is followed for the selection of the stationary phase in chromatography?

Answer:

The stationary phase is selected in such a way that the components of the sample have different solubility’s in the phase. Hence, different components have different rates of movement through the stationary phase and as a result, can be separated from each other.

Q: 12. Describe a method for refining nickel.

Answer:

Nickel is refined by Mond’s process. In this process, nickel is heated in the presence of carbon monoxide to form nickel tetracarbonyl, which is a volatile complex.

Then, the obtained nickel tetracarbonyl is decomposed by subjecting it to a higher temperature to obtain pure nickel metal.

Q: 13. How can you separate alumina from silica in bauxite ore associated with silica? Give equations, if any.

Answer:

To separate alumina from silica in bauxite ore associated with silica, first the powdered ore is digested with a concentrated solution at and bar pressure. This results in the leaching out of alumina as sodium aluminate and silica as sodium silicate leaving the impurities behind.

Then, gas is passed through the resulting solution to neutralize the aluminate in the solution, which results in the precipitation of hydrated alumina. To induce precipitation, the solution is seeded with freshly prepared samples of hydrated alumina.

During this process, sodium silicate remains in the solution. The obtained hydrated alumina is filtered, dried, and heated to get back pure alumina.

Q: 14. Giving examples differentiate between ‘roasting’ and ‘calcination’.

Answer:

Roasting is the process of converting sulphide ores to oxides by heating the ores in a regular supply of air at a temperature below the melting point of the metal. For example, sulphide ores of , Pb, and are converted to their respective oxides by this process.

On the other hand, calcination is the process of converting hydroxide and carbonate ores to oxides by heating the ores either in the absence or in a limited supply of air at a temperature below the melting point of the metal. This process causes the escaping of volatile matter leaving behind the metal oxide. For example, hydroxide of , carbonates of are converted to their respective oxides by this process.

Q: 15. How is ‘cast iron’ different from ‘pig iron”?

Answer:

The iron obtained from blast furnaces is known as pig iron. It contains around carbon and many impurities such as in smaller amounts.

Cast iron is obtained by melting pig iron and coke using a hot air blast. It contains a lower amount of carbon than pig iron. Unlike pig iron, cast iron is extremely hard and brittle.

Q: 16. Differentiate between “minerals” and “ores”.

Answer:

Minerals are naturally occurring chemical substances containing metals. They are found in the Earth’s crust and are obtained by mining.

Ores are rocks and minerals viable to be used as a source of metal.

For example, there are many minerals containing zinc, but zinc cannot be extracted profitably (conveniently and economically) from all these minerals.

Zinc can be obtained from zinc blende calamine , Zincite etc.

Thus, these minerals are called ores of zinc.