Chemical reactions occur when the atomic structure of reactants are rearranged to make new products. The following are signs that a chemical reaction may have occurred:

1.      The formation of a precipitate may indicate that a chemical reaction has occurred. Sometimes, when two liquids are mixed, a solid is formed and settles to the bottom of the solution. The solid that forms is called a precipitate.

2.      Color changes may indicate the presence of a chemical reaction.

3.      Temperature changes are often indicative of chemical reactions. For example, exothermic reaction release energy and make their surrounding feel warmer. Oppositely, endothermic reactions absorb heat energy and make their surrounding feel cooler.

4.      The production of odor may indicate that a chemical reaction has occurred. Just think of the smell of a campfire!

5.      Bubbles may be a sign that a chemical reaction has occurred. Although bubbles from a liquid are due to a physical change of state, bubbles can also be produced by a chemical reaction. For example, yeast in bread can produce bubbles of carbon dioxide that help the bread to rise.

How do you know that a chemical reaction has occurred?

Chemical reactions require reactants and products. During a chemical reaction, reactant concentration decreases and that of products increase, i.e. reactants are converted to products. The occurrence of chemical reaction can be determined by a number of methods. We can check for the concentration of reactants and products over time. If the concentration of reactants is decreasing and that of products is increasing (or is more than what we started with), the reaction has taken place. A number of reactions are exothermic, i.e. they generate energy (such as mixing of solid sodium with water), for such reactions, we can feel the reactor surface and if it is hot (i.e. even without production of flames), reaction has taken place. In some cases, gases are produced as part of products and in such a scenario, we can either see them or smell them and know when the reaction has occurred.

Hope this helps.

How do you know for sure that a chemical reaction has taken place?

A chemical change involves changes in the chemical composition of the reacting species. That is, the chemical composition of the material/s before and after the change are different.

The most certain way to know if a change is a chemical change is to determine the chemical composition of the products. However, since that may not always be possible, we can use indirect methods to confirm the chemical change. There are a number of observations associated with a chemical change and if one of them is there, we can be sure that the change is chemical in nature. Here is a small list of such observations:

• Generation of gases or bubbles
• Changes in temperature
• Formation of precipitate
• Generation of odor or smell
• Change in the color of the solution 

If one observes some of these, one can be fairly sure that a chemical change has taken place.

Hope this helps.