soda therefore contain? Is it possible it may also contain a third element? If so, what?

Experiment 45.

Materials:

Solutions prepared in Experiment 44.
Aluminium foil inch

inch, two or three pieces.

(a) Put a piece of aluminium foil in a test tube of hot water.

(b) Heat the liquid left in the dish in Experiment 44, and put in a piece of aluminium foil. What difference do you observe in the behavior of this liquid and water?

(c) Heat the strong solution of caustic soda prepared in Experiment 44, drop in aluminium foil, and cover the mouth of the test tube loosely with the thumb for a few moments to allow the evolved gas to collect. Test this gas with a flame, immediately after removing the thumb.

What gas is produced by the action of aluminium on caustic soda? Does aluminium act on water under the conditions of these experiments. (See (a).) What element must caustic soda, therefore, contain? Name the elements of caustic soda discovered in this experiment and the preceding one.

The experiments just performed have illustrated the fact that in addition to the alkaline salts there are some other substances which have an alkaline reaction. These are the hydroxides (compounds with oxygen and hydrogen) of the most active metals - sodium hydroxide (caustic soda), potassium hydroxide (caustic potash), and calcium hydroxide (slaked lime, the aqueous solution of which is limewater).

These hydroxides are designated the strong or caustic alkalies in contradistinction to the salts of alkaline reaction, which are called weak or mild alkalies the former being more vigorous than the latter in such actions as characterize alkalies in general; for example, in their effects upon fats, of which we shall learn more later.

Experiment 46.

Test solutions of potassium hydroxide and calcium hydroxide with red litmus paper. Give the common names of these two substances.

CHAPTER XVI

BASES AND BASIC OXIDES

THE hydroxides of metals are called bases. The strong alkalies are therefore bases. These are rather exceptional among bases in being soluble in water. The majority of the bases are insoluble; for example, ferric hydroxide, cupric hydroxide, aluminium hydroxide.

The insoluble base of a given metal (e.g. aluminium) can be obtained as a precipitate by adding one of the soluble bases (sodium hydroxide or potassium hydroxide) to the solution of a salt of the metal (such as aluminium chloride or sulphate).

Dissolve the salts in water in separate test tubes, labeling the tubes. Test a portion of each solution with potassium hydroxide solution and another portion of each with sodium hydroxide. Compare the precipitates obtained where the two soluble bases are added to the same salt. Also compare the precipitates obtained on adding the one soluble base to two salts of the same metal. Save the precipitates for use in Experiment 48.

Experiment 48.

Treat a very small portion of one or two of the solutions with calcium hydroxide solution, using a much larger quantity of this solution than of the sodium or potassium hydroxide.

EXERCISE

1. Write verbal equations for the reactions involved in Experiments 47 and 48, underscoring the names of the precipitates.

2. Rewrite the above equations in symbols, underscoring the formulas of the precipitates.

Basic Oxides

Most of the bases on drying or heating are decomposed into basic oxides and water. Thus:

With cupric hydroxide this decomposition occurs even in the presence of water, as is evident from the change of color which occurs when the blue precipitate is heated.

Experiment 49.

Materials:

Copper sulphate solution.

Specimen of cupric oxide.

To copper sulphate solution in a test tube add sodium hydroxide

solution. Note the color of the precipitate. What substance is it? Heat to boiling. What change occurs? When this substance is dried, it is found to be identical with that obtained by burning copper in oxygen, viz. cupric oxide, CuO. Compare the color of the heated precipitate with that of a specimen of cupric oxide.

The hydroxides of most metals, however, do not show such a color change when they are converted into oxides by the removal of water from their molecules.

In a few instances the basic oxide in the cold readily recombines with water to form the base. A striking instance is that of calcium oxide (quicklime) which takes up water with evolution of great heat in the familiar process of lime slaking (or slacking).

The reaction:

Calcium hydroxide = Calcium oxide + Water

Ca(OH)2

=

CaO + H2O

is thus seen to be a reversible one, running in one direction at high, but in the opposite direction at lower, temperature.

Experiment 50.

Materials:

Quicklime.

Red litmus paper.

Place a small lump of good quicklime (say 10 grams) in a porcelain dish and add as much warm water as the lime will absorb. Allow to stand for a few minutes. What change occurs in the lime? Treat a little of the slaked lime with water in a test tube and test the water with red litmus paper.

Experiment 51.

Materials:

Magnesium ribbon, inch.

Red litmus paper.

Burn a piece of magnesium ribbon. What is the product? Place this product in a dish, add water, and stir for some time. Test the liquid with red litmus paper. Account for the result.

1. Write equations for the reactions of Experiments 49, 50, and

2. Write equations representing the dehydration of, that is,