Here's how it works:

* States of Matter: Chemists use symbols to denote the physical state of substances in a reaction:

* (s) Solid

* (l) Liquid

* (g) Gas

* (aq) Aqueous (dissolved in water)

* Red Boxes as a Visual Cue: While standard symbols are common, some texts or diagrams may use red boxes to visually highlight the gaseous components of a reaction, especially when focusing on gas production or consumption.

Example:

The reaction of magnesium with hydrochloric acid:

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

In this equation, the red box around H₂(g) indicates that hydrogen gas is produced as a product of the reaction.

Note: The use of red boxes to represent the gaseous state isn't universal. If you see a chemical equation with red boxes, it's best to check the context or legend of the diagram to confirm their meaning.