1.Change of odor.

Change of color (for example, silver to reddish-brown when iron rusts).

Change in temperature or energy, such as the production (exothermic) or loss (endothermic) of heat.

Change of composition - paper turning to ash when burned.

Light and/or heat given off.

2.The formation of a gas is the third sign that a reaction may have occurred. The formation of bubbles when two liquids are mixed usually indicates that a gas has formed. A gas can also be formed when a solid is added to a solution.

3.A reaction that absorbs heat is endothermic. Its enthalpy will be positive, and it will cool down its surroundings. This reaction is exothermic (negative enthalpy, release of heat). ... When the reaction occurs, the surroundings will decrease in temperature due to the loss of heat the system absorbed.

4.The identity of the precipitate can often be determined by examining solubility rules. It also occurs in single displacement when one metal ion in solution is replaced by another metal ion. Notice the new solid forming on the bottom of the tube.