Ice chiller
Question: Last night I visited an Indian restaurant with three friends. We
ordered, among other things, a 1-litre glass bottle of sparkling water. On
arrival it looked perfectly normal, but within about 8 seconds of being opened
most of the water had turned to ice. We watched in amazement as the rapid
solidifying started at the top and eerily worked its way right down the bottle.
Needless to say we were dumbfounded. What is the explanation for this bizarre
occurrence?
Answer: The freezing point of water can be lowered by dissolving something
(pretty much anything) in it. Roughly speaking, the solute gets in the way of
the coordinated sticking together of water molecules required to make ice. This
is why salt is spread on roads in the winter.
The case of dissolved carbon dioxide is particularly interesting, because the
gas is forced into water under pressure. When the pressure is released by
opening the bottle, the gas comes out, making little bubbles, and eventually
leaving flat water. Sealed bottles of carbonated water can be cooled below 0
掳C and remain liquid. When the CO2 comes out on opening, the water
is able to freeze, giving the effect you describe.
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John Swain
Department of Physics
Northeastern University
Boston, Massachusetts
Answer: There are three factors that make fizzy water in bottles freeze when
they are opened.
The first is pressure. Because ice is less dense than water, increasing
pressure will lower the freezing point. When the pressure is released the water
is then supercooled and consequently freezes.
The second factor is the adiabatic expansion of gas. When the gas dissolved
in the water is released it expands, work is done and so the gas becomes
cooler.
The third effect is caused by nucleation. Still water can be supercooled
(cooled below its freezing point) and remain liquid if there are no nuclei where
ice crystals may form (and we hope that there are no suspended solids in mineral
water). Then, when the bottle is opened, the gas bubbles provide nucleation
sites.
My team feels that nucleation is probably the main mechanism, though the
other two must contribute to some extent.
Guy Cox
University of Sydney
The four factors given above may work together but not all will be equally
important. We assume that the water in the bottle has been cooled below its
鈥渘ormal鈥� freezing point. The way in which the freezing spreads as a wave from
the top of the bottle is most interesting. It suggests that when the bottle is
opened and the pressure falls, bubbles which form first at the neck of the
bottle provide nucleation sites from which a crystal can grow rapidly
诲辞飞苍飞补谤诲蝉鈥抬诲
One small step
Question: How long will astronauts鈥� footprints on the Moon鈥檚 surface remain
there?
Answer: Footprints in sand on the surface of the Earth will not last long at
all because of the action of wind. But because the Moon has no atmosphere it
might be expected that the footprints would last indefinitely鈥攗nless they
are disturbed by a moonquake produced by a meteorite impact. However, this is
not the case.
In the mid-1970s I worked in the Lunar Physics Group at Birmingham
University, where my speciality was fission track analysis. Fission tracks are
produced in crystal material, such as that found on the lunar surface. On the
Moon, uranium is present as a trace element and some of this undergoes
spontaneous fission, producing fission fragments. The damage that these cause to
the crystalline structure can be revealed by etching the crystal and examining
the fragments鈥� tracks under an optical microscope. The older a specimen is the
more tracks should be revealed, as it has had more time to accumulate
damage.
Research on materials recovered from the Moon鈥檚 surface by the Apollo
astronauts and the Soviet robot missions revealed that the crystals contained
fewer fission tracks than would have been expected, given the age of the
material. This could be explained as an annealing effect. The high surface
temperatures produced during the solar day tend to 鈥渉eal鈥� the damage in the
crystals. This is similar to processes used in the semiconductor industry, in
which crystals of silicon are heated in ovens under controlled conditions to
remove defects.
The big surprise, though, was that this annealing process seemed to have
affected the top metre or so of the lunar surface. Because of the poor thermal
conductivity of the surface layers, one would not expect more than the first
couple of centimetres to be affected. That the effect is felt much deeper than
this implies that the surface material is constantly circulated. This has become
known as the 鈥渓unar gardening effect鈥�. It is thought that expansion and
contraction in the surface layers, caused by the extreme temperature changes
between lunar day and lunar night, lead to a complete turnover of the top metre
every 10 000 years or so.
Assuming the footprints left on the Moon to be approximately 1 centimetre
deep, if the turnover of the top metre takes 10 000 years, it will take 100
years for the footprints to disappear completely.
John Mills
Halesowen, West Midlands
This week鈥檚 questions
Copper conundrum: The Independent newspaper recently stated that 鈥渋f
whisking is something you do often, consider indulging in a copper bowl. The
copper reacts with egg whites to make them fluff up to a greater volume than
whites whisked in other bowls.鈥�
Is this true? If so, what is happening?
Peter Slessenger
Reading, Berkshire
Horse play: In the 19th century, great Clydesdale horses were bred for sale
on the Scottish isle of Lismore. They were ferried to the mainland in a
purpose-built, flat-bottomed boat with a very low freeboard to allow easy
boarding. Walter Weyndling, the Board of Trade Inspector of Ferries, notes in
his delightful Ferry Tales of Argyll and The Isles (published by
Sutton, 1997), that the ferrymen had to induce each horse to 鈥渟tale鈥� (urinate)
before embarking 鈥渙therwise the liquid produce of four giant horses could reduce
stability to the point of capsizing鈥�.
If the urine had been retained in their mighty bladders at horse height, the
effect of this mass on stability might well be hazardous. Releasing the fluid
would obviously lower the centre of gravity鈥攜et it was this decanting
which perturbed the ferrymen. Can anyone tell me why this should make the
flat-bottomed horse ferry less stable?
Ian Morrison
Scottish Institute of Maritime Studies
University of St Andrews