1. Formation of a new substance:
* Change in color: The original substance changes color permanently. For example, rusting iron turns from silvery grey to reddish-brown.
* Formation of a precipitate: A solid forms from a solution, often appearing cloudy or settling to the bottom. For example, when you mix solutions of lead nitrate and potassium iodide, a yellow precipitate of lead iodide forms.
* Formation of a gas: Bubbles form in a solution, or you see the release of a gas. For example, baking soda reacting with vinegar produces carbon dioxide gas.
2. Change in energy:
* Heat release (exothermic): The reaction releases heat, making the surroundings warmer. For example, burning wood or a match.
* Heat absorption (endothermic): The reaction absorbs heat, making the surroundings cooler. For example, dissolving ammonium nitrate in water.
3. Irreversibility:
* Difficult to reverse: The original substance can't be easily recovered by simple physical means. For example, burning wood can't be reversed to get back the original wood.
Important Note: While these signs indicate a chemical change is likely, they aren't foolproof. Some physical changes can also exhibit some of these signs, such as a change in color due to dissolving a solid or a gas being released from a compressed liquid.
