英文科學(xué)讀本 第一冊(cè)·Lesson 20 A Piece of Salt

Lesson 20 A Piece of Salt

Fred, you might let me be teacher to-night, said Willie. "I want to talk about salt."

All right, mate, said Fred, "and I'll go and ask mother for a lump of salt."

Fred was soon back with the things that were wanted.

Now, Norah, said Will, "I want you to put a piece of the salt in the water. That's right. Now stir the water."

Where is the salt? Can you see it?

No, said Norah, "I cannot see it, but I am sure I put it in the water."

Put another piece in and try again, said Will. "Can you see the salt now?"

No, I cannot see it now, said Norah. "What has become of it? Has it melted, Willie?"

Ah, said Will, with a knowing smile, "I thought you would ask that. Now just tell me what you saw, when I put the piece of wax in the spoon, and held it over the fire."

The solid wax became liquid, and flowed about in the spoon, said Norah.

Quite right, said both boys, "that is just what we mean by melting. The solid wax melts when it becomes liquid. We can see it when it is melted. But we cannot see the salt in the water. The salt is not melted."

Teacher says the water loosens and breaks up the salt into tiny pieces, so small that they cannot be seen, said Fred.

You must try to keep in your mind the word, which teacher gave us for this. We say that we dissolve salt in water, because the salt loosens and breaks up in the water, so that we cannot see it. Teacher says to dissolve means to loosen or break up. Don't forget that we can dissolve salt, sugar, soda, alum, and lime.

But, added Will, "we can't dissolve everything, Norah. We can't dissolve stones, chalk, iron, glass, or wood."

SUMMARY

When we put salt into water it breaks up into such little pieces that we cannot see them. We dissolve the salt in the water. It does not melt. We can also dissolve sugar, soda, alum, and lime in water. But we can't dissolve stones, chalk, iron, or wood.