Q: I use salt and vinegar to clean the outsides of my copper pots. I simply smear them over the copper with a paper towel, then rinse. It works instantly but I don't understand how. I remember -- from past chemistry courses -- that acids dissolve away metallic salts. Muriatic acid is used by plumbers to do just that. But vinegar by itself does nothing to copper. What chemical effect does the salt have?

A: When copper is heated, it forms oxides and sulfides with atmospheric oxygen and with the sulfur in natural gas. These compounds are the main cause of copper's discoloration.

Normally, these compounds are quite insoluble both in water and in acid. So they adhere to the metal unless you take a steel wool pad to it, which removes the pigments, after much elbow grease.

The combination of salt and vinegar works instantly and with no labor, however, because salt, which is sodium chloride, forms large molecules known as coordination complexes with the copper ions of copper oxides and sulfides coating the pan's surface. This complexing reaction happens only in acidic solution, so that's why vinegar is necessary.

Q: What makes fruit breads crumble excessively?

A: There are a number of possible causes of the crumbliness in fruit-flavored quick breads:

Inadequate gluten formation -- the major cause of this is poor flour choice. You should use a flour that contains adequate protein. Cake and pastry flours are therefore the wrong flours for quick breads. Instead, use all-purpose or a mixture of half bread, half cake flours.

Freezing -- quick breads that are slightly crumbly when baked become almost impossible to slice or eat in one piece after freezing and thawing.

Oven temperature -- if you bake the breads at a low temperature, excessive evaporation may occur.

Overmixing -- if you mix the batter too long or if you use butter that has melted, the fat may be incorporated too finely and inhibit gluten formation.