Hydrogen, being the simplest element, undergoes various physical changes, which are changes in its physical properties without altering its chemical composition. Here's a breakdown:

1. Phase Changes:

* Solid Hydrogen: Exists at extremely low temperatures, below -259.14 °C (-434.45 °F). It's a white, crystalline solid.

* Liquid Hydrogen: Exists between -259.14 °C and -252.87 °C (-434.45 °F and -423.17 °F). It's a colorless, odorless, and highly volatile liquid.

* Gaseous Hydrogen: The most common form at room temperature and pressure. It's a colorless, odorless, and flammable gas.

2. Expansion and Contraction:

* Like other gases, hydrogen expands when heated and contracts when cooled. This property is used in hot air balloons and other applications.

3. Diffusion:

* Hydrogen has a very low molecular weight, making it the smallest and lightest element. This allows it to diffuse rapidly through porous materials and even through certain metals.

4. Solubility:

* Hydrogen is only slightly soluble in water and other liquids. However, it dissolves readily in some metals, forming metal hydrides.

5. Conductivity:

* Hydrogen is a poor conductor of heat and electricity in its gaseous state. However, its electrical conductivity increases significantly when it's in its plasma state (ionized gas).

6. Viscosity:

* The viscosity (resistance to flow) of hydrogen is very low, especially in its gaseous state.

7. Surface Tension:

* Hydrogen has a very low surface tension, which means it doesn't form strong bonds at its surface.

Key Points to Remember:

* These physical changes don't affect the chemical composition of hydrogen.

* The physical properties of hydrogen are significantly influenced by its extremely low atomic mass and its existence as a diatomic molecule (H₂).

Let me know if you'd like more detail on any specific physical change of hydrogen!