Final answer:
To prevent a helium balloon from floating in the air at 25°C, the helium should be cooled to the ambient air temperature, creating a situation where buoyancy is neutral. If the goal is for the balloon to sink, the helium needs to be cooler than 25°C to increase its density further.
Step-by-step explanation:
To determine the temperature at which a helium balloon no longer floats in the air, we can invoke the principles of buoyancy and the ideal gas law. For a balloon to float, it must displace an amount of air whose weight is greater than the weight of the helium and the balloon itself. As the temperature of the helium gas inside the balloon decreases, its density increases until it becomes equal to the density of the surrounding air. Let's consider a helium balloon at 25°C, which is the air temperature in this scenario. If the helium inside the balloon is cooler than that, the balloon will have a tendency to float due to the lower density of the cooler helium compared to the warmer surrounding air. To stop the balloon from floating, you would need to cool the helium to the ambient air temperature of 25°C, at which point the densities would be equal and the balloon would no longer have sufficient buoyant force to rise.