英文科学读本 第二册·Lesson 07 Soluble Substances

Lesson 07 Soluble Substances

Come and help me, Will. said Fred. "I've got all ready, and I want to show Norah why some things dissolve in water.

I have a tumbler here, filled with water to the brim. I want you to stir it gently, while I put some salt into it—a spoonful at a time. Are you ready? One, two, three, four, five, six. Now we must be careful—seven. I have put seven spoonfuls of salt into the tumbler of water. But if you look you will see some of the salt is now settling at the bottom.

I suppose, said Norah, "the salt that sinks to the bottom is not dissolved."

That's right, said Fred. "The rest of the salt is all dissolved; we cannot see it."

But, Fred, said his sister, "the tumbler was full to the brim at first. If you had put seven spoonfuls of water in, it would have run over the top. How have you been able to put seven spoonfuls of salt in?"

Ah, said Willie, "that's the funny part of it.

The water is porous. As the salt dissolves it breaks up into such tiny morsels, that they can find their way into the pores of the water. When these pores are all filled, the water can dissolve no more salt. The rest sinks to the bottom in a heap.

Now we'll empty this away, and do the same with some lime. said Fred.

He put in seven spoonfuls of the lime, but by the time he had done there was a great heap of wet lime at the bottom. The water too was flowing over the top of the glass.

They waited till the water was clear. Then they poured it out into another tumbler. To make sure there were no solid bits of lime in the water, they made it filter through blotting-paper.

Now look, said Fred, "I am going to breathe into the water through this straw. If there is any lime dissolved in it, the water will turn white like milk as I breathe into it.

See, it looks quite milky already, he added. "The water has got some lime in it. All the lime did not sink to the bottom of the tumbler. Some of it was dissolved. Water dissolves a large quantity of salt, but only a little lime."

SUMMARY

When a substance dissolves in water, it breaks up into little morsels, which find their way into the pores of the water. The water cannot dissolve any more after the pores are all filled.