Note

Student make a list of physical changes.

A chemical change is one that causes a change in the composition of a substance, as, for example, the burning of sugar, when it loses all its physical properties.

Note

Student make a list of chemical changes.

The science of chemistry has to do with chemical changes. The science of physics has to do with physical changes.

In the preparation of food, and in its digestion, assimilation, and excretion from the body, a series of very complex chemical and physical changes occurs.

Among the most important and most abundant of the elements found in the body are oxygen, carbon, hydrogen, nitrogen, phosphorus, and sulphur.

Oxygen is a colorless, odorless, tasteless gas and is the great supporter of life and combustion. It forms one-fifth of the volume of air and eight-ninths of the weight of water. It is a very active element, uniting with all other elements but one. It is more abundantly distributed in nature than any other element and forms about 66 per cent of the human body.

Note

Teacher prepare oxygen and perform some simple experiments to illustrate its properties, using any standard Chemistry as a guide.

Carbon is an odorless and tasteless solid. It is found in nature in a nearly pure state in several different forms. The diamond, hard coal, and graphite, while having vastly different physical properties, are almost pure carbon. All living organisms, both animal and vegetable, contain carbon, and over one hundred thousand carbon compounds have been artificially prepared in the laboratory. Under certain circumstances carbon unites chemically with oxygen, forming carbon monoxide (CO) or carbon dioxide (C02). Heat is given off during this change. Carbon and its compounds form the larger part of fuels and of fuel- or heat-giving foods. Hydrogen is a colorless, odorless, tasteless gas and is the lightest substance known. It burns easily, or unites chemically with oxygen, forming water (H20), heat being given off during this change. Hydrogen burns in the body, but the heat is not intense. Hydrogen, as well as carbon, is found in fuels and in fuel foods; in general, the higher the percentage of hydrogen in a food, the greater the amount of heat it yields when it burns.

Note

Teacher prepare hydrogen and perform some simple experiments to illustrate its properties, using a standard Chemistry as a guide.

Nitrogen is a colorless, odorless, tasteless gas. It forms about four-fifths of the atmosphere. It is an inert gas and forms compounds which easily decompose, a process which, as we shall see later, is due to the action of tiny living organisms called bacteria. Nitrogen is present in the protoplasm, the jellylike content of cells of plants and animals, and is an essential constituent of all living organisms. The human body contains about 2.4 per cent of nitrogen, which is obtained from certain foods that furnish nitrogen in a form that the body can digest and assimilate. Prominent among these foods are meat, eggs, fish, milk, etc.; without a certain amount of such foods the body would undergo nitrogen starvation.

Sulphur is a yellow, combustible solid. It is always present in cell protoplasm in combination with the nitrogen compounds, and is essential for the body. Plants absorb sulphur compounds, or sulphates from the soil, and animals obtain the sulphates which they require from vegetable foods and from meat, milk, eggs, etc. When foods that contain sulphur decompose under the action of bacteria, the sulphur unites with some of the hydrogen present, forming hydrogen sulphide (H2S), the disagreeable odor of which is known in spoiled eggs.

Phosphorus is a highly combustible solid and in the pure state it is exceedingly poisonous. In combination in the form of phosphates, it is essential to every living cell in the body and must be supplied to the body by such foods as contain the phosphates in a form in which the body can assimilate them. Among these foods are meat, milk, egg yolk, wheat, grains, and the legumes.