Soap with water forms:
- (a)Metallotropic liquid crystal
- (b)Thermotropic liquid crystal
- (c)Homogeneous solution
- (d)Lyotropic liquid crystal
Correct — D, lyotropic liquid crystal. Soap molecules are amphiphilic — a water-loving head and an oil-loving hydrocarbon tail. Dissolved in water above a certain concentration they self-assemble into ordered arrangements (micelles and, at higher concentrations, layered or hexagonal phases). A liquid-crystalline phase produced by changing the concentration of such molecules in a solvent is called a lyotropic liquid crystal, so soap in water forms a lyotropic liquid crystal.
- (a)Metallotropic liquid crystal — Metallotropic phases are formed by mixtures that contain both organic molecules and inorganic, metal-bearing components; ordinary soap in water is not one of these.
- (b)Thermotropic liquid crystal — Thermotropic liquid crystals form as a function of temperature in a pure compound (as in LCD displays); soap-in-water ordering is driven by concentration in a solvent, so it is lyotropic, not thermotropic.
- (c)Homogeneous solution — Above the critical micelle concentration soap does not remain a simple homogeneous solution; the molecules aggregate into ordered micellar and liquid-crystalline structures, which is exactly what makes it a liquid crystal.
A liquid crystal is a state between an ordinary liquid and a crystalline solid, with some molecular ordering yet the ability to flow. Lyotropic liquid crystals arise when amphiphilic (surfactant) molecules such as soap are dissolved in a solvent, and their ordering depends on concentration; thermotropic ones depend on temperature instead.
Classify by what drives the ordering. Soap is a surfactant in water, and its micellar and layered ordering appears as the concentration rises — the definition of a lyotropic liquid crystal. Temperature-driven ordering (thermotropic) and metal-bearing systems (metallotropic) do not describe soap-and-water, and it is not a plain homogeneous solution once micelles form.
- Soap is a surfactant, so each molecule has a hydrophilic head and a hydrophobic hydrocarbon tail.
- Above the critical micelle concentration, soap molecules aggregate into micelles and ordered phases in water.
- Lyotropic liquid crystals form from amphiphiles in a solvent and are governed by concentration, whereas thermotropic ones are governed by temperature.
- The same amphiphilic behaviour lets soap trap oily dirt inside micelles during washing.
- Assuming soap simply dissolves to a homogeneous solution — it forms ordered micellar phases.
- Confusing lyotropic (concentration-driven) with thermotropic (temperature-driven) liquid crystals.
- Thinking liquid crystal means only the material inside electronic displays.
NDA/UPSC ask what type of liquid crystal soap-in-water forms, or to distinguish lyotropic from thermotropic liquid crystals.
No directly related past PYQ was found.
- practice — not a real PYQ
Thermotropic liquid crystals change their ordered phase mainly with
- (a)concentration
- (b)temperature
- (c)pressure
- (d)magnetic field
Answer(b) temperature — thermotropic phases depend on temperature, unlike concentration-driven lyotropic ones.
- practice — not a real PYQ
The aggregates formed by soap molecules in water above a certain concentration are called
- (a)crystals
- (b)micelles
- (c)polymers
- (d)emulsions
Answer(b) micelles — clusters with tails inward and heads outward, forming above the critical micelle concentration.