Page 77 - Science Coursebook

5.3 Endothermic processes

Another way to cool down
If you put about 25 cm of water in a beaker and thermometer
3
then stir in three spatulas of potassium chloride,
you will find that the beaker gets cold. In this

case no chemical reaction has taken place. The
potassium chloride has just dissolved. A solution
of potassium chloride has been formed. Potassium potassium glass
chloride is the solute and the solvent is the water. chloride rod

When potassium chloride dissolves in water,
heat energy is taken in (absorbed) from the
surroundings. This is why the beaker feels cold.
This is an endothermic process.
water
Ice melting is another endothermic process. Heat
energy from the environment is taken in as the
solid ice changes to liquid water. Think about
what happens to the particles when water changes
state. The particles in the ice are lined up in rows

and can only vibrate about fixed positions – they energy taken in
cannot move around inside the ice. The forces
between the particles are strong.
As the particles gain energy from the surroundings
they vibrate more and more. The ice begins to
melt. When the particles have enough energy they
can move and overcome the forces holding them ice water
in place. The particles are now able to slide past
one another. The water is now in a liquid state.

Questions
6 Why is ice melting called an endothermic process and not an
endothermic reaction?
A+I 7 Suggest a change of state, other than ice melting, that is an endothermic
process.
A+I 8 When you have been swimming and you come out of the pool, you may feel
cold. Use your understanding of endothermic processes to explain why.
A+I 9 Suggest whether water freezing is an endothermic or exothermic process. Can
you explain your suggestion?

Summary
• An endothermic reaction is a chemical reaction in which energy is
taken in.
• An endothermic process is a process in which energy is taken in,
for example dissolving.
• During an endothermic reaction or process, the temperature decreases.

5 Energy changes 75