Why does iron have to get wet to rust?
Is it good to drink water with your meals?
When iron rusts, it is really slowly burning (combining with oxygen).
If your house is on fire, you throw water on it to stop the burning.
Yet if you throw water on iron it rusts, or burns, better than if you
leave it dry. What do you suppose is the reason for this?
The answer is not difficult. You know perfectly well that iron does
not burn easily; we could not make fire grates and stoves out of iron
if it did. But when iron is wet, a little of it dissolves in the water
that wets it. There is also a little oxygen dissolved in the water,
as we know from the fact that fish can breathe under the water. This
_dissolved_ oxygen can easily combine with the _dissolved_ iron;
the _solution_ helps the chemical change to take place. The
chemical change that results is oxidation,--the iron combining with
oxygen,--which is a slow kind of burning; and in iron this is usually
called _rusting_.[10] But when we pour water on burning wood, the wood
_stops_ burning, for there is not nearly enough oxygen dissolved in
water to combine rapidly with burning wood; and the water shuts off
the outside air from burning wood and therefore puts the fire out.
[Footnote 10: The rusting of iron is not quite as simple as this, as
it probably undergoes two or three changes before finally combining
with oxygen. But the solution helps all these changes.]
Another chemical change, greatly helped by solution, is the combining
of the two things that baking powder is made of, and the setting
free of the carbon dioxid (CO_2) that is in one of them. Try this
experiment:
EXPERIMENT 104. Put half a teaspoonful of baking powder in the
bottom of a cup and add a little water. What happens?
The chemical action which takes place in the baking powder and
releases the gas in bubbles--the gas is carbon dioxid (CO_2)--will not
take place while the baking powder is dry; but when it is dissolved,
the chemical change takes place in the solution.
If you ate your food entirely dry, you would have a hard time
digesting it; and this would be for the same reason that baking powder
will not work without water. Perhaps you can drink too much water with
a meal and dilute the digestive juices too much; certainly you should
not use water to wash down your food and take the place of the saliva,
for the saliva is important in the digestion of starch. But you need
also partly to dissolve the food to have it digest well. Crackers and
milk are usually more easily digested than are plain crackers, for the
milk partly dissolves the crackers, and drinking one or two glasses of
water with a meal hastens the digestion of the food.
_APPLICATION 78._ Explain why paint preserves wood; why iron
will rust more quickly in a wet place than it will either
under water or in a dry place; why silver salts must be
dissolved in order to plate a spoon by electricity.
INFERENCE EXERCISE
Explain the following:
501. There is dew on the grass early in the morning.
502. Cold cream makes your hands and face soft.
503. Glowworms and fireflies can be seen on the darkest
nights.
504. A lake looks gray on a cloudy day and blue on a clear
day.
505. Dried fruit will keep much longer than fresh fruit.
506. If you scratch a varnished surface, you can rub the
scratch out completely by using a cloth wet with alcohol.
507. Soda is usually dissolved in a little water before it is
added to a sour-milk batter.
508. Iron rusts when it gets wet.
509. Peroxide is usually kept in brown bottles.
510. Dry lye may be kept in tin cans, but if the lye is
_moistened_ it will eat the can.