Physical And Chemical Changes

How many changes take place every day in common things!

The burning candle changes from an opaque white solid to a translucent liquid, and then to a mixture of invisible gases. Salt, upon being mixed with water, becomes a clear liquid not distinguishable from water itself. The solid carbon of wood and the gas oxygen unite to form carbon dioxide, a gas quite different from oxygen; and when the action is over, a handful of gray ashes is all the solid sub-stance left. Heat readily changes ice to water, and water to steam.

We are so used to these happenings that they excite in us no wonder; yet, for hundreds of years men have been studying these and similar changes, without finding out all there is to be known about them. So important is the part they play in our everyday work, especially in cooking, that a knowledge of the simpler facts about them is a great help to housekeepers.

These changes are of two kinds. Liquid candle grease returns to the solid form when cooled; dissolved salt may be recovered by evaporating the water; even steam may be collected, condensed, and frozen. No new substance has been formed. The change which has taken place is a phys-ical change. When, however, melted candle grease becomes gaseous, it will not return to its original form ; burnt sugar will never be sweet and white again; water acted upon by sodium is neither water, steam, nor ice. New sub-stances have been formed. In each case a chemical change has taken place. Heat, especially in the presence of moisture, often brings about chemical changes.

Elements And Compounds

Some substances are simple ; that is, they consist of but one thing. Examples : iron, oxygen, carbon. A simple substance is an element. Other substances are composed of two or more elements. Examples : water, carbon dioxide. Into what elements may water be separated (p. 27) ? carbon dioxide (p. 5) ? A substance composed of two or more elements combined is a compound. In a mixture each substance keeps its own properties; in a compound these give place to new properties belonging to the compound. Every chemical change involves either the forming or the decomposition (breaking-up) of a compound, usually both. Many substances may be decomposed by electricity. Tarnish on silver is one of these (p. 43).

The Elements Found In Food; Some Of Their Properties

Foods consist of compounds formed chiefly from oxygen, carbon, hydrogen, and nitrogen, with small quantities of other elements. Oxygen is an invisible, odorless gas. It is a very active element, always ready to unite with other elements to form new compounds. The combining of oxygen with another element is called oxidation. The rusting of metals and the decay of organic matter are slow forms of oxidation. Oxygen forms about one-fifth of the volume of air, eight-ninths of the weight of water, and two-thirds of the weight of the human body. Hydrogen1 is an invisible gas. It will burn, uniting with oxygen to form water. It forms about one-ninth of the weight of water, and one-eleventh of that of the human body. Nitrogen is an invisible incombustible gas. It does not readily combine with other elements, and the compounds into which it enters break up easily. It forms about one thirty-ninth of the weight of the body. Carbon exists as an element in two forms, graphite, the so-called "lead" of pencils, and the diamond. It is most commonly met with in a slightly impure form as charcoal. Of all the elements, no other enters into so many compounds as does carbon. It is contained in all organic substances, as is shown by their blackening (carbonizing) when heated. Food also contains chlorine, a gas when uncombined; phosphorus and sulphur, solid substances, both poisonous when uncombined; and calcium, potassium, sodium, magnesium, and iron, all metals. It is plain that no element by itself is eatable. Nevertheless, chemical compounds of these elements make up our food.

1 The word hydrogen means "water-maker."


A salt is a compound resulting from the union of an acid with one of a class of substances called bases. Commonest among bases are the alkalies (pp. 33 and 108). Common salt (sodium chloride) can be made by adding hydrochloric acid to caustic soda. Calcium salts play the chief part in making water hard. Calcium carbonate causes temporary hardness. Calcium sulphate causes permanent hardness (p. 25).

Action Of Acids On Metals

Acids act on metals, particularly those exposed to dampness and air, forming salts or oxides of the metals. The staining of steel and the corroding of tin and other metal ware by potatoes, fruit, etc., are caused by the action of organic acids in the food. The salts formed in this way are likely to be poisonous. Food not naturally acid may become so by the action of bacteria. (See Sour milk, p. 96.)

Brief Reference List

For further development of topics treated in this section see: -

Snell: Elementary household chemistry.

Vulte : Laboratory notes in household chemistry.

Dodd : Chemistry of the household.

Richards and Elliott : The chemistry of cooking and cleaning.

Brownlee and Others : First principles of chemistry.

Morgan and Lyman: Chemistry.