Here's what happens to the density of a helium balloon when its volume doubles:

Density Decreases by Half

Density is calculated by dividing mass by volume:

* Density = Mass / Volume

When the volume of the balloon doubles, and the mass of the helium gas inside remains the same, the density is halved.

Here's why:

* Mass stays constant: The helium gas inside the balloon doesn't disappear. It's just spread out over a larger space.

* Volume increases: The balloon expands, meaning the helium occupies a larger volume.

Since the mass stays the same while the volume increases, the density must decrease.

Example:

Imagine you have 1 gram of helium in a balloon with a volume of 1 liter. The density is 1 gram/liter. If the balloon expands to 2 liters, the density becomes 1 gram / 2 liters = 0.5 grams/liter.