Junior high school chemistry: the main applications of physical properties of substances are (represented by serial numbers)

1. Lighting candles: Combustion is the process of rapid oxidation of objects to produce light and heat. This is the light produced by the burning of candles.

2. Use balloons filled with hydrogen to lift off during the festival, and hang the slogan: The density of hydrogen is smaller than that of air, resulting in upward buoyancy.

3. wok with pig iron for cooking: iron has good thermal conductivity and fast heat transfer, so cooking will be fast.

4.4. Propelling the launch vehicle by using the energy released by hydrogen combustion: using the huge thrust generated by fuel (hydrogen) combustion, that is, the recoil principle.

So it's 2 and 3.