KEY2CHEM

Observation of Change

Change can be classified as chemical (rearranging chemical bonds to result in a change in chemical composition), physical (rearranging intermolecular forces without a change in chemical composition), or ambiguous. Observations on the macroscopic level provide insight into what change is occurring on the microscopic (particulate) level.

Example 1.

When solid ice melts to liquid water, what changes occur on a microscopic (particulate) level?

A. Covalent bonds between oxygen atoms and hydrogen atoms break.

B. Metallic bonds between water molecules break.

C. Hydrogen bonds between water molecules break.

Solution

C. Hydrogen bonds between water molecules break.

Hydrogen bonds are noncovalent intermolecular forces. In ice or water, they hold adjacent molecules together. When water undergoes a physical change from solid to liquid, the intermolecular forces (but not covalent bonds) break.

Example 2.

Table sugar (sucrose, $$\require{mhchem}\ce{C12H22O11}$$) is dissolved in tea. Which equation best describes this process?

A. $$\require{mhchem}\ce{C12H22O11(s)-> 12 C^{4+}(aq) + 22 H+(aq) +11 O^{2-}(aq)}$$

B. $$\require{mhchem}\ce{C12H22O11(s) -> C12H22O11(aq)}$$

C. $$\require{mhchem}\ce{C12H22O11(aq)-> C12H22O11(s)}$$

Solution

B. $$\require{mhchem}\ce{C12H22O11(s) -> C12H22O11(aq)}$$

Sucrose is a covalent compound that is a nonelectrolyte (will dissolve but does not dissociate into ions). Dissolving sugar in tea is an example of a physical change, which does not break covalent bonds.

Example 3.

Which is an example of a chemical change?

A.

B.

C.

Solution

B.

A chemical change results from the rearrangement of covalent bonds to form a substance with different chemical composition.