英文科學(xué)讀本 第五冊(cè)·Lesson 22 The Boiling Point of Liquids

Lesson 22 The Boiling Point of Liquids

Let us try and learn something more about the heating of liquids this morning, said Mr. Wilson. "We will begin, as before, with the simple experiment of boiling some water in this flask over the Bunsen burner. I will stand the thermometer in the flask, and we will watch it gradually rise, as the water increases in heat.

You will notice that, when ebullition sets in, the thermometer registers 212℉. Instead of taking the flask away as soon as the water boils, we will let it remain in the flame; that is to say, we will continue to add more heat to the water. What does the thermometer say now, Fred?"

It still registers 212°, sir, replied Fred. "The water is still boiling, but it does not seem to get any hotter."

That is so, my boy, said Mr. Wilson. "The boiling water would remain at 212°; it would never rise higher than that.

I will now hold another thermometer, not in the boiling water, but in the steam which rises from it. What do we see? The steam, too, registers 212℉.; and it cannot become hotter than this. The thermometer would not rise higher, however long it might be held in the steam. We will now try another liquid—alcohol or spirits of wine, in place of water. As before, we will put the thermometer in the flask to mark the gradual rise in temperature. The mercury rises in the tube till it reaches about 172℉., and at that point the liquid begins to boil, and pass away as vapor.

No further rise in temperature will take place, and all the time the thermometer continues to register 172°, the alcohol will boil and pass off in invisible vapor. Common ether will boil and pass away as vapor at about 98° Fahr.; in other words, the heat of the sun on an ordinary summer's day is sufficient to boil ether.

I have here some strong brine; let us boil that in the flask now, with the thermometer, as before, to mark the rise in temperature. Watch carefully till you see the liquid boil. Why, what is the matter? The thermometer registers 212°, but the brine is not boiling. See, the mercury is still rising, and yet there is no ebullition. The brine will not boil till the thermometer registers 230℉. Spirits of turpentine must rise to nearly 270℉. before boiling sets in; mercury does not boil below 660℉.

The point at which various liquids boil is called their boiling-point. Thus the boiling-point of water is 212℉.; of alcohol 172°; of ether 98°; of brine 230°; of spirits of turpentine 270°; and of mercury 660℉. I think now it is time for us to have another experiment. I will place this flask, half-filled with water, over the flame, and let it boil, as before. As the water boils, the steam which rises from it will fill the rest of the flask, forcing out the air to take its place. You can see by the violent commotion that the water is boiling, and remember that now the rest of the flask above the water contains, not air, but steam. While things are in this condition I will remove the flame, cork the flask securely, and invert it. The agitation in the water will gradually cease; the boiling is over.

Now watch carefully what happens next. I dip this sponge into cold water, and squeeze it over the upturned bottom of the flask. The boiling at once begins again, although this cooling must have lowered the temperature much below 212°. Let us see what this means. The steam at first took the place of the air which it had expelled from the flask. It exerted a certain pressure on the surface of the water. When the cold water from the sponge cooled the flask, some of that steam was condensed, and fell in drops of water. There was less steam then to press upon the surface and, in consequence, the boiling began a second time. At the ordinary pressure of the atmosphere water does not boil below 212℉., but when that pressure is diminished, water boils at a lower temperature."

Then, sir, I suppose, said Fred, "water would boil at a lower temperature in a balloon, or on the side of a mountain, than it does here, for the pressure of the atmosphere always diminishes as we ascend."

Yes, my lad, replied Mr. Wilson, "you are quite right. On the top of Mont Blanc (15,800 feet high) water boils at about 180℉.; at Quito (11,000feet) the boiling-point is about 194°, and at Madrid (3000 feet) about 207°.

It must be clearly understood, moreover, that these various temperatures cannot be exceeded in any case. On Mont Blanc, for instance, it would be impossible to make the water hotter than 180°, because that is the boiling-point there, and this point is no sooner reached than the water ceases to exist as a liquid, and passes off as steam. Food is cooked with extreme difficulty in such places. Indeed, it cannot be said to be cooked at all, for these low boiling-points do not allow the water to reach a sufficient temperature to extract all the nourishment and flavor from the food. An egg boiled on Pike's Peak would not coagulate, a potato would remain hard.

In deep mines, on the other hand, the atmospheric pressure is greater, and the boiling-point is higher than 212℉.