When a substance undergoes a phase change, such as from a liquid to a solid (freezing) or from a solid to a liquid (melting), there is a transfer of energy involved. In the case of freezing, water molecules lose kinetic energy as they transition from a disordered liquid state to a more ordered solid state. A release of thermal energy—what we typically refer to as heat—accompanies this loss of kinetic energy.
Heat of Fusion for Water
The amount of energy required or released during a phase change is quantified by a thermodynamic property known as the heat of fusion (). For water, is approximately 334 joules per gram.
Simply put, freezing one gramme of water at 0 degrees Celsius releases 334 joules of energy. One gramme of ice at its melting point (0 degrees Celsius) will absorb 334 joules of energy from its surroundings as it melts into liquid water. This energy change is due to the heat of fusion, which is 334 joules per gramme for water.
Calculating Energy Release During Freezing
The amount of energy released during this phase change can be determined using the formula:
- Q is the energy released (in joules).
- m is the mass of the substance undergoing the phase change (in grams).
- is the heat of fusion, which is the amount of energy required to change one gram of a substance from a liquid to a solid state at its melting/freezing point.
For water, the heat of fusion () is approximately 334 joules per gram. Now, let’s calculate the energy released when 65.8 grams of water freeze:
So, when 65.8 grams of water freezes, approximately 21,973.2 joules of energy are released.
Understanding the energy changes during phase transitions like freezing is essential in various scientific and practical applications. For instance, it’s crucial in the design and operation of refrigeration systems. Freezing water in a fridge or freezer absorbs heat from the environment, lowering the temperature and preserving food.
Moreover, the concept of latent heat, which includes the heat of fusion, is fundamental in meteorology. In colder climates, lakes and rivers can freeze over until April. Water turns into ice by absorbing heat from fusion. Lakes take longer to ice over and defrost in the spring because this phase shift requires a lot of energy.
Meteorologists must understand these processes because they affect local climates, seasonal variations, and even the timing of events like the northern river ice breakup, which can have economic and ecological ramifications.
In summary, when 65.8 grams of water freezes, it releases approximately 21,973.2 joules of energy due to the heat of fusion associated with the phase change from liquid to solid. Water molecules transmit energy when they solidify from a disorderly liquid to a solid. Understanding these energy changes is essential in various scientific fields and has practical applications in areas like refrigeration and meteorology.
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