To determine which substance has the greatest kinetic energy at room temperature (~25°C), we can consider the state of each substance at that temperature based on their melting and boiling points.
- Silicon: Melting point of 1,410°C - at 25°C, silicon is solid.
- Helium: Melting point of -272.2°C and boiling point of -268.9°C - at 25°C, helium is in a gaseous state.
- Mercury: Melting point of -38.8°C and boiling point of 356.6°C - at 25°C, mercury is a liquid.
- Methane: Melting point of -182.4°C and boiling point of -161.5°C - at 25°C, methane is a gas.
Kinetic energy generally increases with temperature and is highest for gases compared to liquids and solids. Among the substances listed, helium and methane are gases at room temperature, while mercury is a liquid and silicon is solid.
Between helium and methane, helium has a lower molecular weight and thus, at room temperature, it will have greater kinetic energy compared to methane that has a higher molecular weight.
Therefore, the substance with the greatest kinetic energy at room temperature (~25°C) is helium.