A hydrogen inflated polythene balloon is released from the surface of the earth. As the balloon rises to an altitude up in the atmosphere, it will

1 A hydrogen inflated polythene balloon rises because the density of hydrogen is much lower than the density of the surrounding air, providing buoyancy. 2 As the balloon rises to higher altitudes in the atmosphere, the external atmospheric pressure decreases significantly. 3 The pressure of the hydrogen gas inside the balloon remains relatively higher than the decreasing external pressure. 4 This pressure difference causes the gas inside to expand, pushing the flexible polythene balloon outwards. 5 Consequently, the balloon will increase in size.