What is spreading out?

Harriet opens a perfume bottle at one end of a classroom. A minute later, students at the far end smell it.

Something spread out across the room. In a spontaneous process, the something is matter, energy, or both. Which is it here?

Here, matter spread out. Perfume particles left the bottle and drifted through the whole room.

The room's energy did not spread anywhere new; the classroom stayed at one temperature. The spreading was a spreading of matter.

Now consider a hot pan cooling on a switched-off burner.

The pan's matter stays put. Not one bit of pan leaves.

Its energy is what moves. Thermal energy spreads out of the pan into the kitchen air.

Sometimes both answers are yes. A pot of hot soup steams: water vapor spreads through the kitchen, and the soup's thermal energy spreads into the air.

Matter spreading, energy spreading, or both: identifying which is the first step, every time.

One reading sits near the line. Matter that spreads carries its own energy along. That carried energy does not count as energy spreading. Energy spreading means thermal energy leaving a hot thing for cooler surroundings, the way the cooling pan's thermal energy left the pan. The perfume carried its particles' energy across the room, yet the room's energy did not spread anywhere new. The classroom stayed at one temperature, so the perfume case sorts as matter, not both.

Your turn