In chemical equations, the state of a substance is indicated by adding a symbol in parentheses after the formula. An upward-pointing arrow (↑) indicates that the substance is in the gaseous state.

Here's an example:

```

2H₂(g) + O₂(g) → 2H₂O(l)

```

In this equation, the "g" in parentheses after H₂ and O₂ indicates that they are both gases, and the "l" after H₂O indicates that it is a liquid.