MATTER AND ENERGY

2.5 - Energy

2.5.2 - Endothermic and Exothermic Reactions

When physical or chemical changes (section 2.3) occur, energy is either produced or absorbed. When the process produces energy, it is an exothermic process. When the process absorbs energy, it is an endothermic process.

The melting of snow is an endothermic process. Let's analyze this process. A source of heat causes the phase transition from solid to liquid. In section 2.1 we analyzed the submicroscopic properties between solids, liquids and gases.

The heat energy from the sun causes snow to melt. In the solid form, snow has regular hexagonal shapes. The hexagonal units touch, and are tightly packed in a regular arrangement. The snow absorbs the heat energy of the sun. In the melting process, the sun's heat energy is converted to the liquid water's kinetic energy. As the snow melts, the water molecules increase in kinetic energy resulting in losing their regular hexagonal shapes, and becoming mobile so that the water molecules in the liquid state can glide over each other. In the end, when snow melts to become a puddle of water, an endothermic process has occurred.

Section 3.5
Energy ..p62

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