Again, if you should pour a strong acid over sawdust, it would "char" it, or change it into another substance, _carbon_.

In both of these cases--that of the starch and of the sawdust--what we call a _chemical change_ would have taken place between the acid and the starch, and between the strong acid and the sawdust.
If we looked into the matter more closely, we should find that what has happened is that the starch and the sawdust have changed into quite different substances.

Starches are _insoluble_ in water; that is, although they can be softened and changed into a jelly-like substance, they cannot be completely melted, or dissolved, like salt or sugar.
Sugar, on the other hand, is a perfectly _soluble_ or "meltable" substance, and can soak or penetrate through any membrane or substance in the body.

Therefore all the starches which we eat--bread, biscuit, potato, etc .-- have to be acted upon by the ferments of our saliva and our pancreatic juice, and turned into sugar, called glucose, which can be easily poured into the blood and carried wherever it is needed, all over the body.

Thus we see what a close relation there is between starch and sugar, and why the group we are studying is sometimes called the starch-sugars.
Wheat--our Most Valuable Starch Food.