英文科學(xué)讀本 第三冊(cè)·Lesson 06 Water as a Solvent

Lesson 06 Water as a Solvent

I suppose, said Willie, "teacher only wanted to find out today how much we remember of our early lessons. He went over all the old work."

Yes, said Fred, "and as soon as tea is over I am going to try Norah. I wonder whether she has forgotten all about it."

As soon as they were free, therefore, the boys got a few things together, and Fred began just as their teacher had begun in class.

Look, Norah, he said, "I have here a piece of coal and a piece of salt. If I put them into water can you tell me how they will act?"

Oh yes, said Norah. "The salt will dissolve in the water, but the coal will not dissolve."

But what do you mean by the word dissolve? he asked again.

The word dissolve means to loosen or separate, replied Norah. "The water has the power to loosen and break up, or separate the solid salt into such tiny little particles that we cannot see them. When it is broken up into these tiny particles, we say the salt is dissolved. The water cannot break up the coal. It will not dissolve."

Do you know what we say about the salt because we can dissolve it in water, Norah?

We say, replied Norah, "that the salt is soluble. The coal is insoluble; that means not soluble."

What other substances will dissolve if we put them in water?

Sugar, soda, alum, and lime all dissolve. They are all soluble substances, said Norah.

Now tell us what we say about the water that dissolves these things? said Fred.

We say that water is a solvent for sugar, salt, alum, lime, and any of the things which it dissolves, said his sister.

See, Fred continued, "I can take the coal out of the water just as it was when I put it in. But can I take the salt out in the same way?"

No, the salt cannot be taken out, because it is broken up into very small particles, and the particles are scattered through the water.

Could I get the salt out of the water in any other way? asked Fred.

If you boiled the water, said Norah, "it would all pass away as steam, and the salt would be left behind."

Do you remember, Fred asked again, "that we once filled a tumbler with water to the brim, and then put in salt, a spoonful at a time?"

Oh yes, said Norah, "and I thought you were so clever. I could not make out how you were able to put all that salt in without spilling any of the water."

But you know now, Norah, don't you? asked Will.

Yes, said Norah, "you told me all about it. The water is porous and absorbs the tiny little particles of salt into its pores."

That's right, said Fred. "The tiny dissolved particles, after being broken up and scattered in all directions, fill up the pores in the water. When the pores are all filled the water can hold no more, and the rest of the salt falls in a solid wet mass to the bottom. Remember that water absorbs air and gases in the same way by sucking them up into its pores."

SUMMARY

Water loosens or breaks up salt and other substances into tiny particles too small to be seen. It dissolves them. The dissolved particles are absorbed into the pores of the water. We can get rid of them only by boiling the water. Water will not dissolve some substances. They are insoluble.