UPSC Prelims 2021 Q71: Why Water Is the Universal Solvent
Water can dissolve more substances than any other liquid because
- it is dipolar in nature
- it is a good conductor of heat
- it has high value of specific heat
- it is an oxide of hydrogen
Answer: (a) it is dipolar in nature
Statement-wise explanation
- a: correct — In a water molecule, oxygen pulls the shared electrons more strongly than hydrogen does (Pauling electronegativity 3.44 against 2.20). The molecule is also bent, at about 104.5°, so the two charges do not cancel. The oxygen end carries a partial negative charge and the hydrogen ends carry partial positive charges, which makes water a dipole (dipole moment about 1.85 D). These charged ends surround ions and polar molecules and pull them apart into solution. This is why water dissolves more substances than any other liquid.
- b: incorrect — Water is a poor conductor of heat. Its thermal conductivity is about 0.6 W/m·K, against roughly 400 W/m·K for copper. Even if it conducted heat well, heat conduction has nothing to do with how a liquid pulls a solute apart.
- c: incorrect — Water does have a high specific heat, about 4.18 J/g·°C, which is why it is used as a coolant and why it moderates coastal climates. That property explains how water stores heat, not how it dissolves things. The statement is true but does not answer the question.
- d: incorrect — Water (H₂O) is indeed an oxide of hydrogen, but so is hydrogen peroxide (H₂O₂). Being an oxide does not make a liquid a good solvent. The fact is true but irrelevant.
How to eliminate
This is a 'because' question. Three of the four options state true facts about water: it has a high specific heat, it is an oxide of hydrogen, and it is dipolar. So do not test each option only for truth. Ask whether it explains the effect in the question. Dissolving means pulling solute particles apart and surrounding them. That needs an attraction between the solvent and the solute, and attraction between particles comes from electric charge. Only the dipole option describes charge. Specific heat and heat conduction are thermal properties, and the heat-conduction option is also false, because water conducts heat poorly. 'Oxide of hydrogen' is just a chemical description. The fact to remember is 'like dissolves like': polar solvents dissolve ionic and polar solutes, and water is the commonest polar solvent.
Know more
Why water is the 'universal solvent'
A water molecule is bent, with an angle of about 104.5°. Oxygen is much more electronegative than hydrogen, so electrons crowd towards it. The result is a permanent dipole: δ⁻ on the oxygen and δ⁺ on each hydrogen.
How it dissolves salt: when NaCl enters water, the δ⁻ oxygen ends face the Na⁺ ions and the δ⁺ hydrogen ends face the Cl⁻ ions. Each ion gets wrapped in a shell of water molecules (hydration), and the crystal comes apart. Water's high dielectric constant, about 78.5 at 25 °C, weakens the pull between the separated ions so they stay apart. Polar molecules such as sugar and alcohol dissolve because they form hydrogen bonds with water.
The limit: non-polar substances such as oils, fats and wax have no charged regions to attract water, so they do not dissolve. 'Universal' is a relative term.
| Property of water | Value (approx.) | What it explains |
|---|---|---|
| Dipolar molecule | 1.85 D | Solvent power |
| Dielectric constant | ~78.5 | Keeps ions apart in solution |
| Specific heat | 4.18 J/g·°C | Coolant use; mild coastal climates |
| Thermal conductivity | ~0.6 W/m·K | Poor heat conductor |
| Hydrogen bonding | — | High boiling point; ice floats; surface tension |
Why it matters: water's solvent power carries nutrients in sap and blood and weathers rocks chemically. It also makes water easy to pollute. Dissolved calcium and magnesium salts cause hardness, and arsenic and fluoride dissolve into groundwater. Each property in the table has its own consequence, so UPSC can pair a correct fact with the wrong consequence, as this question does.