Why do changes in internal energy occur: 1) when water is heated in a saucepan,

Why do changes in internal energy occur: 1) when water is heated in a saucepan, 2) when air is compressed in a pump, 3) when snow melts in the mountains.

1) When we heat the water in a saucepan, we give the water additional energy by heating it. This leads to an increase in the speed of movement of molecules, which leads to an increase in internal energy;

2) Gas molecules move chaotically and randomly, colliding with each other. When we compress gas with a pump, we reduce its volume, molecules begin to meet each other and collide more often. This leads to an increase in the speed of movement of molecules and internal energy;

3) Snow is water in a crystal state. When it heats up from the sun, its temperature increases, the molecules begin to move faster, the crystal structure of the snow is destroyed, its internal energy increases and it forms water.