Final answer:
Helium-filled balloons undergo thermal expansion when exposed to different temperatures. In July, when the temperature is higher, the helium inside the balloon expands and the balloon becomes slightly larger.
Step-by-step explanation:
When a helium-filled balloon is exposed to different temperatures in different seasons, such as during January and July, it undergoes thermal expansion due to the change in temperature. Thermal expansion is the process by which the size or volume of a substance changes as its temperature changes. In this case, when the helium inside the balloon experiences higher temperatures in July, it expands and the balloon becomes slightly larger. This is because an increase in temperature leads to increased kinetic energy of the helium particles, causing them to move faster and occupy more space.