Steam is the gaseous form of water. Water and ice change to steam at all temperatures. At 100° C. the steam from boiling water has a pressure equal to the pressure of the atmosphere.

A solution consists of solvent and solute. An emulsion is a mixture in which the suspended substance is so finely divided that it separates itself from the solvent very slowly.

Solubility of a solid is the weight of it that will dissolve in a definite weight, usually 100 g., of water or other solvent.

Crystals are solid bodies that take regular shapes when they separate from solution or from the liquid state. Non-crystalline substances are amorphous.

97. Exercises.

1. If water were most dense at 0° C., what effect would this fact have upon the depth of ice in winter?

2. If a test tube of water is placed in a cup of ether, and a rapid current of air is forced through the ether, the water may be frozen. Why?

3. Which should be kept in the colder room of a storage warehouse, eggs or meat?

4. The sharp edges of a piece fo ice become rounded off even in very cold weather; why?

5. In making "fudges," or the filling of French candies, why is the candy first cooled without being disturbed, and afterwards stirred rapidly?

6. Can you dissolve salt in water, and then recover the salt unchanged by boiling off the water? Can you do the same with sugar? Why?

7. At Quito, in Ecuador, water boils at 90° C., too low for the cooking of potatoes; why? Can you suggest how you could make water boil at 100° C. in Quito?

8. How is salt commonly obtained from salt water? In Northern Russia the salt makers remove much of the water by partly freezing the salt solution, and so save fuel. How is this possible?

EXERCISES

9. Why is it easier to swim in the ocean than in fresh water?

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10. If sugar is dissolved in a cup of tea, does its dissolving affect the temperature of the tea in any way? Explain.

11. If you had some powdered alum, how would you make goodsized alum crystals out of it?

CHAPTER VI

ELEMENTS AND COMPOUNDS

98. Physical and Chemical Changes. If we heat a flat-iron on a stove, it becomes hot. It may even become red hot, so that it gives off heat and light. But if it is taken from the stove, it becomes cold again, and looks just as it did before the heating. A lump of coal thrown into the air comes down again, still a lump of coal. The water of the ocean is changed into steam, then carried away by air currents, and finally falls again as rain or snow. The iron is changed in temperature, the coal and water are changed in position, and the water is changed in physical state (cf. § 69); but the changes do not really alter the iron, coal, and water. We call such changes physical changes.

But if a piece of iron is left in moist air, it rusts (cf. §48). If water is added to quicklime, it combines with the lime, forming "slaked" lime; it is no longer water. If coal is burned, it disappears, as carbon dioxide and steam (cf. § 52). Such changes do alter the nature of substances. They are called chemical changes, because they are studied in the science of Chemistry. Chemical changes are also called reactions.

Digestion consists of the physical and chemical changes that take place in food, in order that it may be taken up by the blood for the use of the body.

ELECTROLYSIS OF WATER

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99. Composition of Water.- By the "composition," or "make-up," of water we do not mean a list of the materials present in a particular sample of water, such as the carbon dioxide, limestone, salt, etc., that are dissolved in a natural water (cf. § 80). What we mean is that pure water, which we obtain by purifying natural water, is still made up of two different substances: hydrogen and oxygen. We say that water is "composed of," or "is a compound of," hydrogen and oxygen.

Water is formed when hydrogen is burned (cf. §§ 52 and 105). Can we reverse the action that takes place in burning, and break up water into hydrogen and oxygen, just as we "decomposed" mercury oxide into mercury and oxygen (cf. § 50)? Water, like mercury oxide, can be broken up by heat; but the temperature required is very high, and the method is hard to carry out. The decomposition of water is easy, if we use the electric current. The operation is called the electro

lysis of water.

Electrolysis means

"loosing," or
or "breaking apart," by
the use of electricity.

100. Electrolysis of Water. The "breaking apart" of water by the electric current may be carried out as follows (Fig. 77):

Electrolysis of Water. The electric current breaks up water, containing a little of an acid, into hydrogen and oxygen.

Two wires from a battery or other source of the current (cf. §§ 150 and 160) pass into a vessel. The vessel contains water and a very little sulphuric acid. The liquid to be changed by the electric current is thus a very dilute sulphuric acid. The wires inside the vessel are

of the metal platinum (cf. § 9), and they have tips of platinum foil. We call the ends of the wires the poles of the battery. If we were to put the two platinum poles together, the current would have a complete passageway, or circuit, without going through the dilute acid. But if we keep the poles apart, the current is compelled to pass through the dilute acid. In the language of the electrician, the dilute acid makes, or closes, the circuit.

While it is carrying the current from one pole to the other, the dilute acid is changed chemically (cf. § 98). What we see is that bubbles of gas arise from the poles. We can collect the gas by placing over each pole a test tube filled with some of the dilute acid. We then see that one test tube collects gas about twice as rapidly as the other. If we put a burning splinter into the gas that is collected the more slowly, the splinter burns more brightly than in air, and if the splinter is merely glowing, it will burst into flame. The gas collected in this tube is oxygen. If we bring a flame near the other gas, the gas takes fire with a slight "pop," or explosion, and then burns with an almost invisible blue flame. This gas is hydrogen. It is so called because it is a part of water. The Greek word for water - "hydor"— appears in many other English words, such as hydra, hydrant, hydraulic, etc.

101. Elements and Compounds. Water is so hard to decompose by heat that men were unable to learn its real nature until 1781. In that year Cavendish, who had prepared hydrogen in 1766, burned hydrogen, and obtained water. The decomposition of water by the electric current was first carried out in 1800. The question now arises: "Can the hydrogen and the oxygen obtained from water be divided up into other substances?" The answer is that they have never been divided by any method used for the purpose. A substance like water, which is not a single kind of matter, but has at least two kinds of matter in it, is called a compound. A kind of matter which we have never been able to break up is called a simple