1. Introduction

We learned that the total energy during a physical or chemical process is conserved in the previous classes. 

Then, why do we hear frequently so much about the energy crisis? 

If energy can neither be generated nor destroyed, we should have no worries! We should be able to do endless activities without thinking about energy resources! 

We can solve this problem if we recollect what else we learned about energy. Energy appears in different forms, and one form of energy can be converted to another. 

For example, 

• If we drop a plate from a certain height, when the plate hits the ground, the potential energy of the plate is mainly converted to sound energy. 
• If we ignite a candle, the process is highly exothermic so that the wax's chemical energy is transformed to heat energy and light energy on burning. 

What are the other products that we get when we light a candle? 

The outcome (total energy) during a physical or chemical process remains the same. 

But, let us consider burning the candle again. Can we somehow combine the heat and light produced along with the reaction products to reclaim the chemical energy in the form of wax?

Chemical energy of wax

Let us take another example. We assume to take $100\ mL$ of water with a $348\ K$ ($75\ °C$) temperature and place it in a room where the temperature is around $298\ K$ ($25\ °C$).

What will happen? Is there any way of obtaining all the heat lost to the surroundings and making the water hot once it has cooled down? 

In each example we consider, we will see that energy in the usable form is dissipated to the surroundings in less usable forms. Hence, any energy source we use to do work is consumed and cannot be used again.