2007-02-07 02:11:32 · 6 answers · asked by Bucky 1 in Science & Mathematics ➔ Chemistry
The liquids in thermometers are chosen for two main reasons. The most obvious one is that you want the liquid to remain a liquid at the temperatures you are likely to measure or you won't be able to use the thermometer. Water freezes at zero degrees C, a temperature we often want to know about (and when it freezes, it expands and can break the glass of the thermometer).
The other reseason is more subtle: you want something that has a large volume change as a function of temperature (its thermal coefficient of expansion). That means the column in the thermometer will change its length more with a given change in temperature and allow you to read the thermometer more accurately.
To answer your question, look up the freezing point of the liquids you mention and the coefficient of thermal expansion.
Mercury is a better choice than alcohol (mercury has a very large value (60 ppm/degree C) and so is used in laboratories, but mercury is toxic while alcohol is much less so. Also alcohol can be dyed red for easier visibility. Mercury, unlike water, dies not expand when it freezes, so it won't break the thermometer bulb.
Check out the references below.
2007-02-07 02:27:00 · answer #1 · answered by Jon K 2 · 0⤊ 0⤋
Both Jon K and Jd are absolutely right about the two essential reasons they have stated. Both of these liquids have much higher coefficients of volume expansion.
There is however, a third reason. Both mercury and alcohol (reddened) levels are easy to observe and READ, when they are within a glass tube. Water meniscus in a fine tube is almost invisible and hence very difficult to read.
Water has other disadvantages like its anomalous expansion (most dense at 4 degrees C), poor conductivity of heat etc.
2007-02-07 02:39:00 · answer #2 · answered by saudipta c 5 · 0⤊ 0⤋
Mercury, also called quicksilver, is a chemical element in the periodic table that has the symbol Hg. A heavy, silvery transition metal, mercury is one of FIVE elements that are LIQUID at or near STANDARD ROOM TEMPERATURE (the others are the metals caesium, francium, and gallium, and the nonmetal bromine). Mercury is used in thermometers, barometers and other scientific apparatus, although the use of mercury in thermometers has been largely phased out in clinical and scientific environments (in favor of alcohol-filled, digital or thermistor-based replacements) due to concerns about the element's toxicity. Mercury is still used in dental amalgam.
Furthermore, water would freeze at 32 degrees, and does not have the same capacity to expand and contract as does mercury and alcohol at the desired ratio. Also, "Mercury will solidify (freeze) at -38.83 °C (-37.89 °F) and so may only be used at higher temperatures. Mercury, unlike water, does not expand upon solidification and will not break the glass tube..."
2007-02-07 02:17:21 · answer #3 · answered by HoneyBunny 7 · 1⤊ 2⤋
they expand and contract with temperature at a greater rate than does water.
Water could be used, (above freezing) but the thermometer would need to be bigger and the temperature lines closer together, making impratical for daily use.
2007-02-07 02:15:17 · answer #4 · answered by Anonymous · 0⤊ 0⤋
THey behave in a way that is perfect for the application
These two fluids expand and contract linearly in the range of temperature we use them for.
Water does not contract when it gets colder, it expands (about 8% of its volume). And it hardly expands when it gets warmed up.
2007-02-07 02:18:43 · answer #5 · answered by Michael Dino C 4 · 0⤊ 0⤋
Because water would freeze at 32 degrees.
2007-02-07 02:20:10 · answer #6 · answered by notaxpert 6 · 0⤊ 1⤋