MC test: Liquid crystals

Multiple choice test on A.4 Liquid crystals

Use the following 'quiz' to test your knowledge and understanding of this sub-topic. As this relates to a sub-topic on the options you may need access to the IB data booklet.

If you get an answer wrong, read through the explanation carefully to learn from your mistakes.

Which word describes mixtures that exhibit the liquid crystal phase at certain concentrations?

Lyotropic liquid crystals in solution show the liquid crystal phase at certain concentrations.

 

Which statement best describes the nematic phase?

In the liquid crystal phase the word nematic refers to the fact that on average the molecules are aligned in the same direction.

 

Which is compound is a biphenyl nitrile?

Biphenyl nitriles must contain a nitrile group, −CN and a biphenyl group, .

 

Which properties of biphenyl nitriles are major contributory factors towards their liquid crystal properties?

I. Their molecules are polar.

II. They are aromatic compounds

III. Their molecules are rigid.

They need to be rigid and polar. The compounds are aromatic but most aromatic compounds are not liquid crystals even though they might be quite stable chemically. It is the way the two phenyl groups are bonded together, not the fact that they are aromatic, that contributes towards their liquid crystal properties.

 

What is the main advantage of liquid crystal displays over other types of electronic displays?

LCDs only work over a limited temperature range and although quite robust are not necessarily more robust or scratch proof than other types of display. They do use much less energy as only a very small electric current is required.

 

Which word describes substances that exhibit the liquid crystal phase over a limited range of temperatures?

Thermotropic is the word that describes substances that exhibit the liquid crystal phase over a limited range of temperatures.

 

Which can function as liquid crystals?

I. soap solution

II. salt solution

III. biphenyl nitriles

Although polar, the particles in salt solution are not rigid enough to form liquid crystals.

 

Which label can only apply to a mixture?

A solvent and a solute must be involved in lyotropic liquid crystals as the property is dependent upon the concentration.

 

Which are important attributes of liquid crystal molecules?

I. chemically stability

II. be able to orientate quickly

III. be polar

All three are important (together with the fact that they must be stable in the liquid crystal phase over a suitable range of temperature).

 

The first liquid crystal to be discovered was cholesteryl benzoate.

Which functional groups are present in cholesteryl benzoate?

I. biphenyl

II. ester

III. alkenyl

Cholesteryl benzoate contains a phenyl group but not a biphenyl group.

 

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