MPEMBA EFFECT

 HOT WATER FREEZES FASTER THAN COLD WATER.

Mpemba was a secondary school student in Tanzania in 1963 who had the fortune of re-discovering some interesting physics during one of his cookery lessons.

Having boiled some milk for making ice cream, his class were told to let the mixture cool before putting it in the refrigerator. Mpemba, however was anxious of ensuring himself a space so put his mixture in straight away.

Mr. Mpemba founder of mpemba effect.

The other students waited and put their mixtures in later, after they'd cooled down. Having noted the time his ice cream entered the freezer compared with the rest of his class, Mpemba realised his mixture had frozen significantly faster than everyone else's.

He came away with the simple observation that "hot liquids freeze faster than cold liquids".

His science teacher told him this was impossible and he must have got mixed up. So why exactly is this impossible?

In a subsequent year at high school Mpemba was taught about Newton's law of cooling in science: the rate at which a body cools is proportional to the temperature difference between that body and its surroundings.

Any set of cooling curves plotted from such a function will never cross, no matter what the initial starting temperature. So a curve which starts at a higher temperature will never undercut a curve starting at a lower temperature and will therefore always take longer to cool. Varying the parameter k on the other hand could well cause graphs to cross. But this parameter is determined from some initial conditions, if both systems are not identical in such things as geometry or arrangement, with the exception of starting temperature, then it is hardly appropriate to compare cooling times for different initial temperatures.

 However, Mpemba was undeterred by a theory which did not seem to support his observations: he asked a friend who sold ice cream in a nearby town who told him he routinely used hot mixtures because they froze more quickly.
Still persisting with this, in 1969 a visiting academic from University College in the capital called Dr Osborne came to visit Mpemba's school and he jumped at the opportunity to quiz him about this apparent violation of Newton's Law. Thankfully he didn't dismiss it outright, and upon returning to Dar es Salaam, he instructed a lab-assistant to carry out an experiment to see if hot water would freeze more quickly than cold water.

The lab-assistant reported the hot water had frozen first, but not to worry, "I'll keep on repeating the experiment until we get the right result." After several attempts it seemed Mpemba was right - hot water would freeze faster than cold water.