This set of Solid State Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “High Pressure and Hydrothermal Methods”.
1. Which of the following statements is true for hydrothermal methods?
a) It uses temperature above its cooling point
b) It uses temperature below its cooling point
c) It uses temperature above its boiling point
d) It is not at all related with the temperature
Explanation: Hydrothermal methods utilize water under pressure and at temperatures above its normal boiling point as a means of speeding up the reactions between solids. The water performs two roles. The water-as liquid or vapour-serve as the pressure transmitting medium.
2. Hydrothermal methods are particularly suited for the synthesis of phases that are_________
a) Stable at lower temperature
b) Unstable at lower temperature
c) Stable at higher temperature
d) Unstable at higher temperature
Explanation: Some or all the reactants in hydrothermal method are partially soluble in the water under pressure and this enables reactions to take place in, or with the aid of, liquid and/or vapour phases. Under these conditions, reactions may occur that, in the absence of water, would occur only at much higher temperatures.
3. Hydrothermal method can be a useful technique for___________
a) Growing liquid CO2
b) Growing amorphous solid
c) Growing single crystals
d) Growing Gaseous particles
Explanation: Hydrothermal method can be a useful technique for growing single crystals by arranging for a suitable temperature gradient to be present in the reaction vessel, dissolution of the starting material may occur at the hot end reprecipitation at the cooler end.
4. Which of the following is correct for hydrothermal method. Hydrothermal method need to be carried out in________________
a) Open container
b) Closed container
c) In a test tube
d) In conical flask
Explanation: Hydrothermal methods must be always carried out in a closed vessel, that is why the pressure and temperature relations of water should be kept in such a way that the volume must remain constant, it is a very important factor.
5. What is the critical temperature of water in ◦C?
Explanation: The critical temperature of water is 374◦C, below this temperature, two fluid phases, liquid and vapour, can coexist. Above this temperature only one fluid phase, supercritical water, ever exists.
Explanation: The design for hydrothermal method is a tube of steel, closed at one end. The other end has a screw cap with a gasket of soft copper to provide a seal Alternatively, the ‘bomb’ may be connected directly to an independent pressure source, such as a hydraulic ram.
7. In hydrothermal method, the bomb connected directly to an independent pressure source is known as____________
a) Hot seal
d) Cold seal
Explanation: In hydrothermal method the bomb may be connected directly to an independent pressure source, such as a hydraulic ram, this is known as the ‘cold seal’ method. The reaction mixture and an appropriate amount of water are placed inside the bomb which is then sealed and placed inside an oven at the required temperature.
8. What is the work of mineralizer in the hydrothermal method?
a) It increases the amount of water
b) It speeds up the reaction
c) It increases the solubility of reaction mixture
d) It helps in forming the cold seal
Explanation: For the growth of single crystals using hydrothermal method, it is often necessary to use mineralizer, a compound added to the aqueous solution that speeds up its crystallization.
9. How does the mineralizer operate in the hydrothermal method?
a) Decreasing the concentration of the solvent
b) Increasing the concentration of the solvent
c) Increasing the solubility of the solute
d) Decreasing the solubility of the solute
Explanation: Mineralizer usually works by increasing the solubility of the solute through the formation of soluble species that would not usually be present in the water.
10. The normal graphite structure has coordination number 3, however on typical transformation u at applied pressure and temperature it becomes diamond with a coordination number of_______
Explanation: Phases synthesized at high pressures tend to have higher densities than phases synthesized at atmospheric pressure and this sometimes gives rise to unusually high coordination number hence at 130 kbar pressure and 3000◦C graphite undergoes transformation and becomes diamond with coordination number 4.
11. Which one of the following is high pressure polymorphs of SiO2?
Explanation: Stishovite is one of the high pressure polymorphs of SiO2 which is formed at pressure 100 to 120 kbar, it has rutile structure this contains octahedrally coordinated silicon.
Sanfoundry Global Education & Learning Series – Solid State Chemistry.
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