Question

what kind of change does a substance undergo without altering the molecules or atoms that are present?

Answer 1

Physical change does not alters the molecule or atoms that are present.

Physical changes are those changes that do not alter the identity of the molecules or atoms that are taking part in the chemical reaction. Or we can say that physical changes are changes in which no bonds are broken or formed. This means that the same types of compounds or elements that were present  at the beginning of the change are there at the end of the change as well. Because the ending materials are the same as the beginning materials, the properties (such as color, boiling point, etc.) will also be the same. Physical changes also  involve moving molecules around, but not changing them.

Whereas Chemical changes occur when bonds are broken and are formed between molecules or atoms. This means that one substance with a certain set of properties (such as melting point, color, taste, etc) is turned into a different substance which have different properties. Chemical changes are frequently harder to reverse than physical changes.

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