| Date | May 2013 | Marks available | 1 | Reference code | 13M.3.sl.TZ1.C3 |
| Level | SL | Paper | 3 | Time zone | TZ1 |
| Command term | State | Question number | C3 | Adapted from | N/A |
Question
Liquid crystals are sometimes used in the construction of “smart windows”.
Smart windows are milky white as their randomly arranged liquid crystals scatter light. When a voltage is applied, the liquid crystals align in the same direction. The light then passes through them without scattering, making the windows transparent.
State the property of the liquid-crystal molecules that allows them to align when a voltage is applied.
List two substances that can behave as liquid crystals.
Distinguish between thermotropic and lyotropic liquid crystals.
Thermotropic liquid crystals:
Lyotropic liquid crystals:
Markscheme
polarity / presence of dipole (moment);
Any two for [1]
graphite
cellulose
(spider) silk
DNA
biphenyl nitriles
soap
Kevlar
Thermotropic liquid crystals:
pure substances and exhibit liquid-crystal properties in a certain temperature range;
Lyotropic liquid crystals:
solutions and exhibit liquid-crystal properties in a certain concentration range;
Award [1 max] for thermotropic pure substances and lyotropic solutions.
Award [1 max] for thermotropic in a certain temperature range and lyotropic solutions in a certain concentration range.
Examiners report
The majority of candidates recognized polarity as the property of liquid crystals in part (a) and were able to give two substances that could act as liquid crystals in part (b).
The majority of candidates recognized polarity as the property of liquid crystals in part (a) and were able to give two substances that could act as liquid crystals in part (b).
Many candidates knew that thermotropic liquid crystals worked within a temperature range and lyotropic liquid crystals work within a concentration range in part (c), but only few candidates distinguished between the two types fully.
