In a chemical equation, the symbol $\Delta$ written above the reaction arrow indicates that:

1. Certain reactions need heat to be supplied from outside to take place. 2. This is shown by the sign $\Delta$ written above the arrow that indicates the direction of the reaction. 3. A catalyst or temperature condition is written below or above the arrow as text, not as $\Delta$. 4. A gaseous product is shown by $\uparrow$ and a precipitate by $\downarrow$, so those options are wrong. _Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 3 "Chemical Reactions and Equations", p.41_