英文科學(xué)讀本 第二冊(cè)·Lesson 06 Soluble

Lesson 06 Soluble

We had another lesson today, said Fred, "about things which dissolve and things which melt."

Let me join you for a few minutes, said his father, "I have here some loaf-sugar broken up fine. I will put some of it into this tumbler of water; the rest into this iron spoon. Fred shall stir the water in the tumbler. I will hold the spoon over the fire. Now watch. I think we can make it all clear.

Look, he went on, "the pieces of solid sugar in the spoon are changing into liquid sugar. We can see it flow about in the spoon. This sugar is melted.

Your sugar, Fred, cannot be seen in the water, although we know it is there. It is dissolved.

Thank you, father, said the children. "That makes it very plain."

Teacher gave us a new name for things that dissolve, said Fred. "All things that dissolve are said to be soluble."

Yes, said Willie, "and when we dissolve any of them, we make a solution."

Teacher showed us the meaning of these three hard words in another way, said Fred. "I think I know clearly now what a solution is. I will try and do it as he did it.

Here is some chalk, crushed up into powder. I will put a spoonful of it into this glass of cold water. Now we will stir it up well. Can you see the chalk?

Yes, said Norah, "the water looks thick and white like milk."

The tiny little bits of solid chalk are floating about in the water, said Fred. "They will not dissolve. If we stir it all night, the chalk will not dissolve.

We cannot see the salt, sugar, and other things when they dissolve. The water looks quite clear. It is a solution. This chalky water is as thick and white as ever. If we leave the glass to stand for a time, all the wet solid chalk will sink to the bottom, and the water will become clear, and may be poured off. But there will not be any chalk in it. All the chalk will be left at the bottom. If we dissolve a substance in water, we cannot pour away the water, and leave that substance at the bottom."

SUMMARY

Things that dissolve are said to be soluble. When they dissolve they make a solution. If we dissolve a substance in water, we cannot pour away the water, and leave that substance at the bottom, as we can chalk. Chalk will not dissolve in water.