英文科学读本 第二册·Lesson 08 Starch

Lesson 08 Starch

What is this, Norah? asked Fred.

It is some of the starch that mother uses to make your collars stiff. said Norah.

Well, I want to have a chat about starch this evening. said Fred.

Watch what happens when I put some of it into this tumbler of water.

It sinks to the bottom, and lies there just as we put it in, said Norah.

Wait while I stir it up well, said Fred. "Now tell us what you see."

The water is thick and milky-looking now, as it was when we put the chalk into water. said Norah.

Stir it a little more, said Fred. "Perhaps we can make it clear then.

No, he went on, "we cannot make it clear with all our stirring. What does that mean, Norah?"

The starch will not dissolve in the water, said Norah. "It is not soluble. The little pieces of solid starch are floating about in the water. It is this which makes the water thick like milk."

Now, we will put some of this thick liquid into a basin, and pour boiling water on it, said Fred. "The rest we will stand aside in the tumbler."

Why, the boiling water seems to swell it up, said Norah, "into a jelly."

Yes, said Will, "that's just it. The hot water breaks up the little solid bits of starch that are floating about. Pour some more boiling water on it, Fred, while I stir it with the spoon.

See now, Norah, he added, "the jelly has become a thin clear liquid. The starch is dissolved at last."

Oh, I see, said Norah;"the starch will not dissolve in cold water, but it dissolves in boiling water."

Now, look at the other starch in the tumbler of cold water, said Fred.

Why, the water is quite clear now, said Norah. "Where is the starch?"

Look at the bottom of the tumbler, cried Willie. "It has all sunk down there. If we pour off the water, we shall leave all the starch at the bottom. None of it is in the water, because it will not dissolve in cold water."

SUMMARY

Starch is not soluble in cold water. Like chalk, it sinks to the bottom, and we can pour the water away. We can dissolve starch in boiling water.