The freezing point of soda is slightly lower than the freezing point of water, which is 32 degrees Fahrenheit. Carbon dioxide is added to soda to give it its fizz, and carbon dioxide lowers the freezing point of water.
If the soda contains sugar, this ingredient also lowers the freezing point, and the mixture will freeze around 28 degrees. Different formulas of soda also change the freezing point. For example, a diet soda without sugar will have a higher freezing point than soda with the normal amount of sugar, but it will still need to reach a temperature lower than 32 degrees to freeze.