【单选题】
When water is heated until it boils， bubbles of gas appear and rise through the
hot liquiD、When an electric current passes through water in a process called
electrolysis (電解)， bubbles of gas appear and rise through the liquiD、Superficially
(表面地)， the two events appear same.
If the gas from the boiling water is examined， its properties are found to be the
same as those of the water， Thus， if the steam is cooled to room temperature， a
liquid is formed which is indistinguishable from the original water. When， however，
the gas from the electrolysis equipment is cooled to room temperature， it remains a
gas rather than becoming a liquiD、Nor will it， at zero degrees centigrade or below，
turn to solid， as will the gas from boiling water， These two processes， boiling and
electrolysis， have clearly resulted in products with quite different properties.
In boiling， the gas does not represent a new substance， but only a different
state of the original substance.Electrolysis， on the other hand， has generated a
product which is a new substance， or possibly a mixture of new substances.A、change
in state as represented by going from a liquid to a gas without the production of new
substances in called a “physical change”。 However， when a process takes place that
produces new substances， this is called a “chemical change” or a “chemical
reaction”。 In a chemical reaction， the initial substances are replaced by a new set
of substances or products.