Tea leaves contain tannins, a loose collection of chemicals that give tea much of its flavor and body, and especially that astringent, puckering effect in the mouth.
They dissolve in water to form a clear solution, as long as the water isn’t too cold or slightly alkaline. Your cloudiness occurs when some of the tannins in the hot tea fall out of solution (precipitate) as tiny solid particles when the tea is cooled. Cloudiness can also form when certain tannins react with the caffeine in the tea.
Tannins are present to some extent in most plant materials but are particularly abundant in oak galls (abnormal growths on oak trees); certain barks, woods, and roots; and nut shells.
All tannins are soluble in water, but how much of them can dissolve in a given amount of water (their solubilities) depends on the temperature of the water and upon its acidity or alkalinity. When hot water is used to make strong tea (the usual first step in making iced tea), it extracts most of the tannins from the leaves. Then, when the solution is cooled down with ice cubes, all those tannins cannot stay dissolved, and they fall back out as fine, suspended solid particles that give the tea a cloudy appearance.
Tannins are more soluble in acidic solutions, so that when an acid such as lemon juice is added to tea, any solid particles of tannin will dissolve and the cloudiness will clear up.
Also, if the tea is initially brewed with hard water—that is, with water containing dissolved calcium or magnesium salts—these minerals can react with the tannins to form relatively insoluble complex chemicals that show up as flotsam and jetsam.
If your water is hard, then, try adding a little lemon juice to clear up any cloudiness. Or switch to a different tea, because some teas, such as Assam and Darjeeling, are richer in tannins and thus more prone to cloudiness than others, such as Ceylon.
In chemistry, the opposite of an acid is called a base. Acids and bases neutralize each other. But because base is a word with many common meanings (more than a dozen each as noun and adjective), I use the words alkali and alkaline in this book instead of base and basic. Strictly speaking, however, the word alkali should be reserved for the very strong bases sodium hydroxide (lye) and potassium hydroxide.
The words tannin and tannic acid are often used interchangeably, but not by chemists and other finicky types. Tannic acid is a specific chemical compound, a high-molecular-weight penta-m-digalloyl-glucose, a.k.a. gallotannic acid, with the formula C76H52046 . On the other hand, the word tannins refers to a whole class of complex plant chemicals that just happens to include tannic acid. They are generically called tannins, not because of any particular chemical similarity (although they are mostly what are known as polyphenols), but because they have been used since prehistoric times for tanning hides: converting raw animal skins into leather in order to improve their durability and resistance to heat, water, bacteria, and fungi.
Tannin polyphenols turn hides into leather by reacting with proteins in the skins to form insoluble adhesive-like substances that bind the protein fibers tightly together. In this tight, dry form, the hide is much stronger and more durable than the raw skin.
Tanning your own hide is a completely different state of affairs. Soaking your body in strong tea or extract of boiled oak galls is not recommended, but exposure to sunlight will induce your skin to produce the dark pigment melanin. So-called self-tanning lotions (they don’t tan themselves; they tan you) usually contain dihydroxyacetone, or DHA, a colorless chemical that reacts with amino acids in the outermost cells of the epidermis (the stratum corneum) to produce a variety of dark reaction products.