The Mpemba Effect: An Overview

Cian McElhinney Émer JonesJonathan McKeonCian Tuohy

What is the Mpemba Effect?

The Mpemba effect is the observation that, in certain specific circumstances, warmer water freezes faster than colder water.

Has never been conclusively explained and baffles scientists.

Several physical conditions have been associated with the effect.

History and Origin:

Phenomenon observed by Aristotle, Francis Bacon and René Descartes.

Aristotle observed ‘antiperistasis’; ‘the supposed increase in the intensity of a quality as a result of being surrounded by its contrary quality’.

Term ‘Mpemba effect’ originated in the 1960’s.

Erasto B. Mpemba:

Tanzanian high-school student.

Noticed effect while doing a school cookery assignment in 1963.

Enquired about the effect during a seminar given by Dr. Denis G. Osborne.

Osborne was sceptical at first but researched and confirmed Erasto’s findings.

The duo published the combined results of their research in 1969.

Definition Of “Mpemba Effect”

“Hot water freezes quicker than cold”

As “Mpemba Effect” isn't very well defined or limited to a specific definition, there are many variables which we fell is one of the main reasons it hasn’t been scientifically proven to date.

Variables

Method Of Freezing Definition of Freezing Container Type & Size Method of taking temperature

Method Of Freezing

Slow freezing Non-frost-free refrigerator (-14°C) Frost-free refrigerator (-14°C) Ice container with salt (-6°C)

Fast freezing Cold liquid (e.g. Liquid Nitrogen -196°C) Cold outside temperature (North/South,

Pole −70°C)

Definition of ‘Frozen’:

Point at which water forms a visible surface layer of ice.

Point at which the entire volume of water becomes a solid

Point at which the temperature ,at any point, reaches 0°C

Point at which the temperature of entire volume is 0°C or lower.

Container Type & Size

Type Use of plastic/glass/Pyrex or metal. As all these materials have different

thermometric properties and physicals (conductivity and type of surface)

Size Changes surface area in contact

with air/container

Method of taking temperature

Digital IR, Thermocouple probe -accurate-

Non-Digital Mercury, Alcohol thermometer -not

as accurate- Thermometer/probe may influence

freezing as they may move or have ice form around them.

What causes the effect?

There is no generally accepted cause.

Causes we proposed: Insulating effect of frost/ice & Convection

currents. Hotter liquids Molecules. Evaporation of the warmer liquid.

-Loss of mass -Change in pressure

Quality of water (Solutes)- impurities Supercooling observed more in cold water

Insulating effect of Frost/Ice and Convection currents

The cooler water will to freeze from the top, reducing further heat loss by radiation and air convection

The warmer water will freeze from the bottom and sides because of water convection.

Igloos: The Insulating Effect of Ice

Temperature outside (approx. 57°C)Inside can still be quite comfortable.

Hotter liquids Molecules

Hotter liquids molecules are more excited than colder ones.

When the water molecule is excited it vibrates and becomes unstable,

It then wants to release energy to become stable.

Evaporation of the warmer liquid:-Loss of mass

As the warmer liquid evaporates its mass decreases. -This may have been the case for Erasto Mpemba.-

But there has been several further experiments using a coating of oil or a lid on the container to prevent loss of mass by evaporation.

Evaporation of the warmer liquid: -Change in pressure

“Liquid freezes at higher temperatures with higher pressure.”

Covering the container will mean there will be different pressure levels in each container.

Container with the hotter liquid may have a greater pressure.

Triple point of water

The combination of pressure and temperature at which liquid water, solid ice, and water vapour can coexist in a stable equilibrium occurs at exactly 273.16 K (0.0098 °C) at atmospheric pressure.

The change in pressure can change this point.

Quality of water (Solutes)- impurities

“Increasing the number of dissolved particles lowers freezing point.”

Boiling the water may remove impurities such as bacteria and other objects that may be suspended or dissolved in the liquid.

Supercooling

Pure water normally freezes at 273.15 K (0 °C) but it can also be "supercooled" at standard pressure down to 231 K (−42 °C).

Our Experiment:

Objectives To demonstrate the effect To investigate the possible causes of

the effect

Variables Method of freezing Definition of ‘frozen’. Container type and size Method of measuring temperature

Method:

Began with three beakers of water at different temperatures. (1=3.9°C 2=19.2°C 3=35.2°C)

Immersed the beakers in a trough of slush (Ice & salt at -6°C).

Used electronic thermometers and an automatic graphing device to record temperature of beakers and slush.

Determined freezing point using a light and a camera.

Observations

12 min – Beaker 1 froze on surface only.

17 min – Surface Tension on Beaker 2+3 “skin” of the water curving around probes.

34 min – Beaker 2 instantaneously froze

38 min – Beaker 3 instantaneously froze

Weights at end of experiment

Beaker 1 – 40.75-32.5=9.25 Beaker 2 – 44 - 32.5 =11.5 Beaker 3 – 43.25-32.5=10.75

Conclusion:

We successfully demonstrated the Mpemba effect.

The initially warmer water samples froze before the five-degree sample.

The five-degree ice sample weighed the least.

We feel that this effect is due to a number of reasons but the main one is “The Insulating effect of Frost/Ice”

Videos

But “The Insulating effect of Frost/Ice” has been proven to not be the main cause as seen in 1st video.

http://www.youtube.com/watch?v=xOVZV6DxaRs @ 1.20, 1.59 & 2.30 min.