Think you know water? Which of these statements are correct?
- Water can turn to steam at freezing point.
- Ice can form at room temperature.
- Water doesn’t have to boil at 100 degrees celcius.
- Hot water can freeze faster than cold.
- Water can “remember” what was dissolved in it when that substance is removed.
You drink it, cook with it, bathe in it and more than 60% of you is comprised of it, but how well do you really know water?
Chances are you have made tea or coffee and you know it boils to steam at 100 degC (212 degF). Similarly you know it freezes into ice cubes at 0 deg celcius (32 degF).
But water is one of the most mysterious liquids around. Physicists and chemists are still discovering just how strange water really is.
What is water?
Water is comprised of molecules made up of one central oxygen and two hydrogen atoms (H2O) in a bent “V” formation.
Atoms are made up of a nucleus orbited by electrons. Electrons carry a negative charge. The nucleus has a positive charge to attract and maintain the electrons. Oxygen has a strong positive charge, and hydrogen a weaker one.Thus in a water molecule, the central oxygen atom is very strong and pulls electrons from the hydrogen such that they spend more time closer to the oxygen (the centre of the “V”) and less time around the edges of the hydrogen (the ends of the “V”). This leaves the ends with a slight positive and the middle with a slight negative charge. These slight charges are enough to allow weak “hydrogen” bonds to set up between adjacent molecules. It is these hydrogen bonds which give water it’s amazing surface tension.
To see these bonds at work, pour water into a glass and also into a transparent plastic container. If you look carefully through the side of the glass, the top of the water forms a gentle “U” shape – and is higher at the sides of the glass. This is called a meniscus. It forms because glass also contains slightly charged molecules and thus water “sticks” to the sides using hydrogen bonds. Plastic does not contain charged molecules and thus a meniscus does not form. The water level in plastic should appear flat.
Ice, the anomaly
Think about a glass of iced water. If you watch the weather channel, you know that hot air rises and cool air sinks. Similarly the water at the bottom of a swimming pool is cooler than the top. This is because warm things are less dense than cold (when warm, atoms have more energy to push away from each other and thus lower the density). This is why gases generally rise and solids generally sink. Yet ice cubes (at 0 degrees celcius) float on warmer water. In fact water is most dense at 4 degC.
Water is almost unique in being less dense as a solid than a liquid, but this very property has an enormous effect on life on Earth (and no, I don’t just mean ice-skating). Because ice floats, ponds, rivers, lakes and oceans freeze at the top first. This leaves an insulated layer of water which stays warmer, allowing fish to survive harsh winters and ice ages.
So lets look at each of our initial “weird” statements in turn:
Water can turn to steam at freezing point
This is a chemical process called sublimation. Rather than melting then continuing to heat until water boils into steam, some water molecules escape straight to gas form. The reverse can happen when frost forms – gaseous water vapour converts directly to solid ice, without becoming liquid. This is true of many substances, not just water.
Ice can form at room temperature
Relatively recent research has shown that water appears to crystallise into nano-scale ice bridges in a very thin (a couple of atoms deep) film of water. Experiments suggest that this may require the presence of a tiny electrical current. The electrical currents needed are so small that they could well be created in nature, in rock crevices and the like. Perhaps your garden pond is “freezing” even in summer.
Water can boil at many temperatures
Altitude and air pressure can affect the boiling point of water. A boiling point is reflective of the forces required to force the molecules apart. At high pressures (where a substance is more compressed), more energy is required. Thus the higher the pressure, the higher the boiling point. At the top of Mt Everest, where altitude reduces air pressure, water boils at approximately 69 degC. On a stormy day where air pressure is up, water can boil slightly higher than normal. A pot containing a lot of water will compress water at the bottom slightly and thus this water will take a slightly higher temperature to boil.
Additives can also vary the boiling point of water. A few lower the temperature (eg. alcohol) but most increase it (such as salt). Just think of the explosive boiling of milk.
Hot water freezes faster than cold.
In 1969 a Tanzanian science student (Erasto Mpemba), running late with an assignment to make icecream, put hot milk into the freezer and watched it freeze faster than cool milk. He was not the first to notice such a phenomena – indeed Aristotle and a number of other notable scientists over the centuries have mentioned the same thing. It seems ridiculous – and also to violate Newton’s law of cooling.
Cooling in general involves the transfer of heat energy from the warmer body (water) to the cooler (air). The greater the difference in temperatures between the substances, the faster the hotter object cools (Newton’s law). However hotter water has more energy in it than cold and thus has more to lose before it can freeze. Thus theoretically, hot water should take longer to cool than cold.
Mpemba was tenacious enough not to be dissuaded by his class teacher’s logic and brought it to the attention of a physicist – Denis Osborne. Testing by later scientists verified that Mpemba was correct. In some cases it is true – hot water does freeze faster than cold water.
How can this be?
The exact mechanisms are not clear but it appears that a number of factors may be at work. The first part is to see the two containers of water as separate entities. They are not identical and even when the hotter water cools to the initial temperature of the cooler, it may not continue to chill in the same way as the cooler container did.
Firstly hot water evaporates faster than cold. Evaporation causes cooling and thus when compared to the cooler water, the hotter, evaporating water will become colder, faster. Evaporation also reduces the total quantity of water which needs freezing. Thus it may reduce the time needed to cool.
Secondly, heating water can change the chemical composition of water. Water contains a number of impurities, and heating water may alter the mixture of gases dissolved in the water. This could alter the density, the freezing temperature and the rate of cooling of the water.
Thirdly, convection may be at work. If the container holding the hotter and cooler water is a little insulating, the water will primarily cool from the surface. When water starts to cool, it will sink, creating currents of water. Hot water will cool faster at the surface (due to evaporation) and this water will move downward, leading to variability in the temperature of the water. As the hotter water is cooling faster, convection currents will be greater in it than currents in the cooler water. Thus the cooler water will maintain a more steady temperature.
As both containers cool, if they reach a point when the average temperature of each container of water is the same, the water which started hotter should have more heat at the top and less at the bottom of the container. In comparison, the water which started cooler will have a more uniform temperature. Heat can transfer out from the warm top level faster than it can from the more uniform water and thus at the point where the average temperatures of both containers are identical, the previously hotter water can continue to cool faster than the previously cooler water.
Finally, there is a phenomena called supercooling. While water may usually become ice at 0 degrees, this is due to particles “seeding” the formation of ice crystals. Without such a seed, water can actually be cooled below freezing temperature. At some point, water will spontaneously form ice. However, without a seed, this can vary. Hotter water may supercool – that is form an ice-like structure a higher temperature than cooler water. ie. Cooler water may need to reach a lower temperature to spontaneously freeze. The freezing points of the two containers may not be the same.
Other possibilities include differences in the airflow around the hot and cold water containers and differences in the surrounding conditions.
While still a point of argument, a combination of these factors proably accounts for this strange property of water.
Water remembers what was dissolved in it when the substance is removed
Some homeopathic remedies involve diluting a solution so much that there is no trace of the original compound added to water. The claim is that this can cure a number of ills. It seems ridiculous – once you have diluted a solution to that extent, the solution remaining should just be plain old water.
However, one Swiss chemist is finding evidence that water actually remembers what was dissolved in it. By taking a salt solution and diluting it extensively, he made it likely that there were no salt molucules left in the container used. Let’s call this a non-salt solution as there is no salt in it anymore. He then froze this non-salt solution and blasted it with radiation. As it warmed, the ice gave off a glow from the energy stored from the radiation. The glow was different when a non-salt solution was used to when pure water was used. The structure of the ice crystals were different when salt had been included in the solution at a previous point in time.
This does not necessarily mean that the homeopathic medicines work however!
Auerbach, D (1995) Supercooling and the Mpemba effect: When hot water freezes quicker than cold, American Journal of Physics : 63(10), pp. 882-885
Cho, CH Surjit, S and Robinson, GW (1996) An Explanation of the Density Maximum in Water, Phys. Rev. Lett. 76, 1651
Choi, E-M Yoon, Y-H Lee, Y and Kang, H(2005) Freezing transition of interfacial water at room temperature under electric fields. Physical Review Letters 95:085701.
Jeng, M (2006) The Mpemba effect: When can hot water freeze faster than cold? American Journal of Physics:74, Issue 6, pp. 514-522
Jinesh, KB and Frenken, JWM (2006) Capillary Condensation in Atomic Scale Friction: How Water Acts like a GluePhys. Rev. Lett. 96 166103
Miranda, PB Xu, L Shen, YR and M. Salmeron (1998) Icelike water monolayer adsorbed on mica at room temperature. Physical Review Letters 81:5876.
Rey, L (2003)Thermoluminescence of ultra-high dilutions of lithium chloride and sodium chloride, Physica A 323 67–74.Sack, NJ and Baragiola, RA (1993) Sublimation of vapor-deposited water ice below 170 K, and its dependence on growth conditions, Phys. Rev. B 48, 9973 – 9978
Sheng, HP and Huggins, RA (1979) A review of body composition studies with emphasis on total body water and fat, American Journal of Clinical Nutrition, Vol 32, 630-647
Velasco, E Mederos, EL Navascués, G (Comment on) An Explanation of the Density Maximum in Water, Phys. Rev. Lett. 79, 179
With thanks to UnitedCats for suggesting the topic.