Answer
These are likely chemical changes:
Bubbles were produced when iron was placed in acid.
Two liquids were combined, and a solid appeared.
A white substance turned blue when water was added.
Explanation:
Chemical changes or reactions involve the transformation of the substances into other substances. For example, hydrogen and oxygen react together to form water. The water, formed by the chemical combination of hydrogen and oxygen, is a completely different substance to the two former. So, you can drink and enjoy water, but not hydrogen and oxygen, which are two completely different substances.
On the other hand, a physical change modifies the physical properties but not the composition of the substances.
You can analyze the changes of the choices in this way:
a) Salt crystals were crushed into a fine powder.
Crushing the crystals changes the size of grains, but do not change the composition of the compounds: the crystals have the same kind of substances, none substance has been transformed. So, this is a physical change.
b) Bubbles were produced when iron was placed in acid.
The formation of bubbles is a sign that a new substance,a gas, has been formed. So, it is a chemical change.
c) Two liquids were combined, and a solid appeared.
Since no solids were involved in the combination, the appearace of a solid, makes to think that a new kind of substance has been formed, product of a chemical reaction. So, this is a chemical change.
d) A piece of metal melted when it was heated.
Meting is a physical change. The composition of the metal does not change, just the position of its particles and so some physical properties.
e) A white substance turned blue when water was added.
The formation of a different color suggests the appearance of a new different substance. This is the producto of a chemical reaction.
