1. Formation of a New Substance: This is the most fundamental indicator. If you start with one or more substances and end up with something entirely different, a chemical change has happened. This new substance will have different properties (color, smell, melting point, etc.) than the original substances.
* Example: Burning wood. Wood (cellulose) reacts with oxygen to form ash, carbon dioxide, and water. The ash, carbon dioxide, and water are completely different substances from the original wood.
2. Production of Light or Heat: Many chemical reactions release or absorb energy, often in the form of light or heat.
* Example: Burning a match. The chemical reaction releases energy as heat and light.
3. Production of Gas Bubbles: If a reaction produces gas bubbles, it's a strong indicator that a new substance has formed.
* Example: Mixing baking soda (sodium bicarbonate) with vinegar (acetic acid). The reaction produces carbon dioxide gas, which you see as bubbles.
Important Note: While these indicators are strong clues, they aren't foolproof. Some physical changes might also show one or two of these indicators (like boiling water producing bubbles). However, the formation of a new substance is the most reliable indicator of a chemical change.
