1. Solid sulfur reacts with oxygen gas to form sulfur dioxide gas:

This reaction is represented by the chemical equation:

```

S(s) + O2(g) → SO2(g)

```

In this equation, "S(s)" represents solid sulfur, "O2(g)" represents oxygen gas, and "SO2(g)" represents sulfur dioxide gas. The reaction is exothermic, meaning that it releases heat.

2. Sulfur dioxide gas reacts with oxygen gas to form sulfur trioxide gas:

This reaction is represented by the chemical equation:

```

2SO2(g) + O2(g) → 2SO3(g)

```

In this equation, "2SO2(g)" represents two molecules of sulfur dioxide gas, "O2(g)" represents oxygen gas, and "2SO3(g)" represents two molecules of sulfur trioxide gas. This reaction is also exothermic.

Both of these reactions involve the breaking and forming of chemical bonds, which is what characterizes a chemical change.