英文科學(xué)讀本 第三冊(cè)·Lesson 09 Water in other Forms

Lesson 09 Water in other Forms

Look, Fred, how the steam is puffing out of the spout of the kettle, said Norah, as they sat round the fire.

Yes, dear, I put the kettle there myself, said Fred. "I wanted you to notice it, for we are going to have a talk about the steam now."

What would happen if we left the kettle on the fire?

The water, replied Norah, "would all boil away, and the kettle would be left dry."

But what does that mean?

The water would be changed into steam, said Norah.

I think, said Fred, "I had better explain it as teacher did. He says that steam is only water in another form. The heat breaks up the water into such extremely small particles, that they are lighter than the air itself. They therefore rise in the air and mingle with it."

But what becomes of it in the air, Fred?

The air is very porous, replied Fred. "It absorbs into its pores these little floating particles, just as water absorbs salt."

You have seen mother hang out the wet clothes on washing day, and you know that after a time she is able to take them in quite dry. What has become of the water? I will tell you. The water in the wet clothes on the line has been broken up into tiny particles—not of steam, but of something like steam. We call it vapor.

These little particles of water vapor, like the particles of steam, are so light that they rise in the air, and are at once sucked up or absorbed into its pores.

We say that the water evaporates. We mean that it changes into vapor.

I have seen the water evaporate after a shower on a very hot day, said Will. "The vapor rises in a cloud from the wet pavements, and in a few minutes the stones are quite dry. On a cold day, too, the pavement dries up, but not so quickly as in the hot summer weather."

But you see, said Fred, "if it dries up at all, it proves that even in the cold weather the water evaporates. Let me explain it to you. It is the air which carries off the water, and it does this because it is porous and absorbent."

Sometimes the air is so very dry that its pores contain little or no vapor. When this is the case the air becomes very thirsty. It will break up the water into tiny particles and suck them at once into its pores.

The drier the air is, the quicker will be the evaporation. Clothes will dry on the line in cold weather as quickly as in hot, if the air is dry.

When the pores of the air are already loaded with water vapor, it is very little use to hang the clothes out. They will not dry. The air cannot take in any more water, because its pores are already full. No evaporation can go on. The water remains in the clothes.

SUMMARY

Steam is water in another form. The heat breaks up the water into tiny particles lighter than the air. They rise and float away in the air. The air absorbs them into its pores. Vapor is something like steam. Some amount of vapor rises even in cold weather. Dry air can evaporate and absorb more moisture than damp air.