Page 152 - Primary Five Science

Chemical or Physical change?

Chemical reactions release or absorb heat or other energy or

may produce a gas, odor, color or sound.

In some cases, it may be hard to tell whether a chemical or

physical change occurred.

For example, when you dissolve sugar in water, a physical
change occurs. The form of the sugar changes, but it remains

the same chemically (sucrose molecules). However, when you

dissolve the salt in water the salt dissociates into its ions (from

NaCl into Na and Cl ). So, a chemical change occurs.
+
-
In both cases, a white solid dissolves into a clear liquid but the

processes are not the same.

The Similarity between chemical and biological change is that;

 Both are irreversible

 Both form new substances