1. Production of Gas: Bubbles forming, fizzing, or a noticeable odor can indicate that a gas is being released as a product of the reaction. Think of baking soda reacting with vinegar or burning wood.
2. Formation of a Precipitate: A solid substance appearing from a solution, often a different color, signals a chemical reaction. For example, mixing solutions of lead nitrate and potassium iodide produces a yellow precipitate of lead iodide.
3. Change in Color: A significant change in color, not simply due to light reflection, usually points to a chemical change. Think of rust forming on iron (brown) or leaves changing color in autumn.
4. Change in Temperature: A chemical reaction can either release heat (exothermic) or absorb heat (endothermic). You'll feel the change in temperature with your hand or measure it with a thermometer. Think of burning fuel (exothermic) or dissolving salts in water (endothermic).
5. Change in Odor: A new, distinct smell appearing often suggests a new chemical compound has formed. Think of the characteristic smell of burnt toast or spoiled milk.
Important Note: These are indicators, not absolute proof of a chemical change. Sometimes, a physical change can also exhibit some of these signs, like the melting of ice (change in state) or the dissolving of sugar (dispersion, not formation of a new substance).
To confirm a chemical change, you'd need to look for evidence of new substances being formed. This can be complex, but it generally involves analyzing the composition of the original substances and comparing it to the composition of the resulting substances.
