CHEMICAL AND PHYSICAL CHANGES
Chemical Changes. Since many of the changes which cooking produces in the different food materials are of a chemical nature, it is well to consider what constitutes a chemical process. This idea may perhaps be best conveyed by a few experiments and illustrations, the materials for which may be easily obtained.
Exp. with Cream of Tartar and Bicarbonate of Soda. Mix two teaspoons of cream of tartar with one of bicarbonate of soda, in a little warm water. A union of the two substances follows and they neutralize each other; that is, the cream of tartar is no longer acid, and the soda is no longer alkaline. Owing to the power of chemical affinities a separation or breaking up of these compounds takes place, and new substances, carbonic acid and rochelle salts, are formed out of their constituents. The effervescence which is seen is caused by the escape of the carbonic acid.
Exp. with Hydrochloric Acid and Soda. Put a few drops of chemically pure hydrochloric acid into a little water; then add soda. A violent effervescence will follow. Continue putting in soda until this ceases, when the reaction should be neutral. Test it with litmus-paper. If it turns blue litmus-paper red, it is acid; if red litmus-paper blue, it is alkaline. Add acid or soda, whichever is required, until there is no change produced in either kind of litmus-paper. The results of this experiment are similar to those in the first one, namely, carbonic acid and a salt. In this case the salt is sodium chlorid or common salt, which is in solution in the liquid. Evaporate the water, when salt crystals will be found.[3]
Oxid of Iron. A piece of iron when exposed to the weather becomes covered with a brownish-yellow coating, which does not look at all like the original metal. If left long enough it will wholly disappear, being completely changed into the yellowish substance, which is oxid of iron, a compound of oxygen and iron, commonly called iron rust.
Burning of Coal. A piece of coal burns in the grate and is apparently destroyed, leaving no residue except a little ashes. The carbon and hydrogen of the coal have united with the oxygen of the air, the result of which is largely the invisible gas, carbonic acid, which escapes through the chimney.
Formation of Water. Water is formed by the union of two invisible gases, hydrogen and oxygen. It bears no resemblance whatever to either of them. Its symbol is H2O.
All these are examples of chemical changes.
Definition of Chemical Change. Chemical changes or processes may be defined as those close and intimate actions amongst the particles of matter by which they are dissociated or decomposed, or by which new compounds are formed, and involving a complete loss of identity of the original substance.
Physical Changes. Mix a teaspoon of sugar with an equal amount of salt; the sugar is still sugar, and the salt remains salt; and they may each be separated from the mixture as such.
Water when frozen is changed from a liquid to a solid; its chemical composition, however, remains unchanged.
Water converted into steam by heat is changed from a liquid to a gas, but chemically there is no difference between the one and the other. Steam, water, and ice are forms of the same substance, the difference being physical, not chemical, and caused by a difference in temperature.
Lead melted so that it will run, and the solid lead of a bullet, are the same thing.
These illustrate physical changes.
Definition. When substances are brought together in such a way that their characteristic qualities remain the same, the change is called physical. It is less close and intimate than a chemical change. The transition from one state into another is also frequently only a physical change, as is seen in the transformation of water into steam, water into ice, etc.