Flint, wood, and iron, like chalk and starch, will not dissolve in water. They are insoluble. Water is a solvent for sugar, salt, and other things, because it dissolves them. Soda is a solvent for fat and oil.
Soap
"I didn"t think," said Willie, "that greasy oil- bottle and the soda could have anything to do with soap. Did you, Fred?""No," said Fred, "I didn"t. Suppose we have our lesson all over again now."As soon as tea was over Fred said, "Do you remember that greasy oil-bottle, Norah?""Oh yes," said Norah. "The soda dissolved the oil,although the water would not dissolve it.""Now look," said Fred; "this bottle is half-full of oil. Smell it and see that it is oil. I have got here a strong solution of soda. What does that mean?""A solution of soda is some soda dissolved in water," said Norah. "I suppose you call it a strong solution, because you have put a lot of soda in it.""Now I will fill the bottle up with the solution," said Fred, "and well give it a good shaking. Tell me what you see.""The oil seems to be mixing with the Soda," said Norah.
"Quite right," said Fred. "If I shake it a little more, the soda will dissolve all the oil. It will form a new substance.
