What property is inversely proportional to temperature in gases, leading to an upward force on the hot gases?

The property that is inversely proportional to temperature in gases is density. As the temperature of a gas increases, the kinetic energy of the molecules also increases, causing them to move apart. This results in a decrease in the density of the gas because density is defined as mass per unit volume. When gas is heated, the expansion leads to fewer molecules in a given volume, thus decreasing its density.

This decrease in density relative to the surrounding cooler air creates a buoyancy effect. Hot gases, being less dense than the cooler gases around them, experience an upward force, causing them to rise. This principle is essential in understanding fire behavior, particularly in how smoke and hot air rise during a fire scenario.

The other properties listed, such as viscosity and pressure, do not have the same direct inverse relationship with temperature and do not primarily explain the upward force on hot gases. Viscosity relates to the resistance to flow and changes in temperature can affect it, but it does not define the buoyancy effect. Pressure is influenced by temperature according to the ideal gas law, but it does not directly account for the upward movement of gases in the same manner as density does.