KEY2CHEM

Change and Interparticle Interactions

A physical change is one that changes only intermolecular interactions (not breaking chemical bonds). A chemical change results in a change in composition, which means breaking and forming new chemical bonds (intramolecular interactions). Some changes (such as dissolving a salt in water) can be classified as both since the strength of the interactions being broken is similar to the strong chemical bond strength.

Example 1.

Which is a physical change?

A. $$\require{mhchem}\ce{H2O(g) -> H2O(l)}$$

B. $$\require{mhchem}\ce{2 H2O(g) -> 2 H2(g) + O2(g)}$$

C. $$\require{mhchem}\ce{H2(g) -> 2 H(g)}$$

Solution

A. $$\require{mhchem}\ce{H2O(g) -> H2O(l)}$$

A physical change breaks interparticle forces but a chemical change breaks chemical bonds to form new substances.

Example 2.

Which is a chemical change?

A. $$\require{mhchem}\ce{O2(g) -> O2(aq)}$$

B. $$\require{mhchem}\ce{C(s) + O2(g) -> CO2(g)}$$

C. $$\require{mhchem}\ce{CO2(s) -> CO2(g)}$$

Solution

B. $$\require{mhchem}\ce{C(s) + O2(g) -> CO2(g)}$$

A chemical change breaks chemical bonds and results in formation of a new substance.

Example 3.

Physical changes occur when _________ interactions are disrupted.

A. intermolecular

B. bonding

C. intramolecular

Solution

A. intermolecular

A physical change does not change the composition of the substance, meaning no chemical (intramolecular) bonds are broken.