Final answer:
A double bond causes a deviation from the ideal bond angle of 120 degrees by repelling the single bonds more strongly, resulting in a smaller bond angle.
Step-by-step explanation:
The presence of a double bond affects the Cl-C-Cl bond by causing a deviation from the ideal bond angle of 120 degrees. In compounds with a double bond, the double bond repels the single bonds more strongly than the single bonds repel each other. This results in a smaller bond angle between the Cl-C-Cl atoms.