.

1. How would you test a water to detect the presence of organic matter?
2. Name some mineral impurities often found in water. 3. Describe the
test for chlorids; for sulphates; for iron; for lime; for magnesium. 4.
Of the two classes of impurities found in water, which is the more
harmful? 5. Name three ways of purifying waters known to be impure, and
tell which is the most effectual.

Experiment No. 49

Testing for Nitrites in Water

To 50 cc. of water in a small beaker add with a pipette 2 cc. of
naphthylamine hydrochloride and then 2 cc. of sulphanilic acid. Stir
well and wait 20 minutes for color to develop. A pink color indicates
nitrites.

REAGENTS USED

Sulphanilic Acid.--Dissolve 5 gm. in 150 cc. of dilute acetic acid;
sp. gr. 1.04.

Naphthylamine Hydrochloride.--Boil 0.1 gm. of solid [Greek:
a]-amidonaphthaline (naphthylamine) in 20 cc. of water, filter the
solution through a plug of absorbent cotton, and mix the nitrate with
180 cc. of dilute acetic acid. All water used must be free from
nitrites, and all vessels must be rinsed out with such water before
tests are applied.

1. Would a water showing the presence of nitrites be a safe drinking
water? Why? 2. What are nitrites? 3. What does the presence of nitrites
indicate? 4. Are small amounts of nitrites, when not associated with
bacteria, injurious?

REVIEW QUESTIONS

CHAPTER I

GENERAL COMPOSITION OF FOODS

1. To what extent is water present in foods? 2. What foods contain the
most, and what foods the least water? 3. How does the water content of
some foods vary with the hydroscopicity of the air? 4. How may changes
in water content of foods affect their weight? 5. Why is it necessary to
consider the water content of foods in assigning nutritive values? 6.
How is the dry matter of a food determined? 7. Why is the determination
of the water in a food often a difficult process? 8. What is the ash or
mineral matter of a food? 9. How is it obtained? 10. What is its source?
11. Of what is the ash of plants composed? 12. What part in plant life
do these ash elements take? 13. Name the ash elements essential for
plant growth. 14. Which of the mineral elements take the most essential
part in animal nutrition? 15. In what form are these elements usually
considered most valuable? 16. Why is sodium chloride or common salt
necessary for animal life? 17. How do food materials differ in ash
content? 18. Define organic matter of foods. 19. H