SSC
The phenomenon in which hot water may freeze faster than cold water under certain conditions is called:
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Answer: Option A — Mpemba effect
Detailed Solution
The Mpemba effect is the observation that, under certain conditions, hot water can freeze faster than cold water, even though it seems that hot water should take longer because it has to cool down further first.
The effect is named after Erasto Mpemba, a schoolboy in Tanzania. In 1963, while making ice cream at school, he noticed that a hot milk-and-sugar mixture placed in the freezer froze faster than a cooler mixture. His teacher and classmates laughed at him, but he continued to investigate. Later, when the physicist Dr Denis Osborne from University College, Dar es Salaam, visited his school, Mpemba asked him about it. Osborne tested it and found that it could happen. They published a paper on it together in 1969.
The phenomenon had been noticed much earlier. Aristotle, in the 4th century BCE, wrote that heated water freezes faster. Francis Bacon and René Descartes also mentioned it.
Scientists still debate why the Mpemba effect happens, and whether it is reliably reproducible. Possible explanations include:
Evaporation: hot water evaporates faster, reducing the amount of water that has to freeze.
Dissolved gases: hot water holds less dissolved gas, which may change how it freezes.
Convection: hot water may develop stronger convection currents, transferring heat away faster.
Supercooling: cold water may supercool more (stay liquid below 0°C) before freezing.
Frost and contact: a hot container may melt the frost below it and make better contact with the freezer surface.
The effect depends strongly on the experimental conditions, and some careful studies have not found it.
Why not the other options?
- ✗The Joule-Thomson effect is the change in temperature of a gas when it expands through a valve; it is used in refrigeration and liquefying gases.
- ✗The Tyndall effect is the scattering of light by colloidal particles.
- ✗The Leidenfrost effect is when a liquid droplet skates on a very hot surface over a layer of its own vapour.
Quick tip
Remember: Mpemba effect = hot water freezing faster than cold (Erasto Mpemba, Tanzania, 1963); Tyndall = light scattering by colloids; Leidenfrost = drops dancing on a hot pan.
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Matter & Its States — Preparation
This question tests your understanding of Matter & Its States, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers matter and its classification: the states of matter, changes of state (melting, evaporation, sublimation and condensation), elements, compounds and mixtures, solutions, colloids and suspensions, and methods of separating mixtures. Exam questions often ask about examples of sublimation (camphor, naphthalene and dry ice), which substances are colloids (milk, fog and blood), and which method separates which mixture, such as chromatography for the colours in ink and fractional distillation for crude oil. Know the difference between a physical change and a chemical change. Practise the matter and its states questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.