Problem 14 - Chemistry The Molecular Nature of Matter

Question

In places like Saudi Arabia, freshwater is scarce and is recovered from seawater. When seawater is boiled, the water evaporates and the steam can be condensed to give pure water that people can drink. If all the water is evaporated, solid salt is left behind. Are the changes described here chemical or physical?

Step-by-Step Solution

Answer

The changes described for seawater (boiling, evaporating, condensing, and leaving solid salt behind) are physical changes, because no new substances with different chemical properties are formed.

Step 1: Identify the Changes

List the changes that are mentioned: seawater boiling, water evaporating and steam condensing, and salt being left behind after all the water has evaporated.

Step 2: Define Physical Change

Define physical change as a type of change where the form of matter is altered, but one substance is not transformed into another.

Step 3: Define Chemical Change

Define chemical change as a type of change that involves a chemical reaction, with the formation of new substances with different properties.

Step 4: Compare with Definitions

Compare the changes described for seawater with the definitions of physical and chemical changes.

Step 5: Conclude the Type of Change

Determine whether the changes are chemical or physical based on whether new substances with different properties are formed.

Key Concepts

Seawater Desalination

Seawater desalination is the process of removing salt and other minerals from saltwater to produce water that is fit for human consumption, agriculture, and other needs.

Desalination can be achieved through various methods such as distillation, reverse osmosis, and electrodialysis. In regions like Saudi Arabia, where freshwater resources are limited, desalination plays a crucial role in providing a steady supply of potable water. The most widely discussed desalination process is distillation, wherein seawater is heated until it boils.

The boiling creates steam, which is then cooled and condensed back into a liquid form that is free from salt and impurities. This method works due to the different boiling points of water and salt. While the water converts into steam, the salt remains in the solid state, allowing for easy separation.

Boiling Point and Condensation

Understanding the concepts of boiling point and condensation is essential in the context of desalination.

The boiling point is the temperature at which a liquid turns into vapor. This occurs when the vapor pressure of the liquid equals the atmospheric pressure. For pure water at sea level, this is typically 100 degrees Celsius (212 degrees Fahrenheit). Condensation is the reverse process, where vapor cools down and changes back into a liquid state.

When seawater is heated to its boiling point during desalination, the water turns to steam leaving the salt behind, as salts have a much higher boiling point. Upon cooling, the steam undergoes condensation and turns back into liquid water, which is now desalinated.

Physical Change Definition

A physical change refers to an alteration of the state or appearance of a substance, without changing its chemical composition.

Physical changes usually involve a change in energy and can be reversible or irreversible. Examples of physical changes include boiling, melting, freezing, and condensation. None of these processes result in the production of a new substance, instead they simply change the form that the original substance takes.

In the desalination process, both the boiling of water into steam and the subsequent condensation of that steam back into water are prime examples of physical changes.

Chemical Change Definition

A chemical change takes place when a substance undergoes a chemical reaction and transforms into one or more different substances with new chemical properties.

Chemical changes are typically irreversible, because they involve the breaking and forming of chemical bonds. This type of change often results in the observation of certain indicators such as color change, heat production or absorption, formation of a precipitate, and/or the production of gas.

During the desalination process, since the composition of water and salt do not change and no new substances are formed, the process is characterized as a physical, not a chemical change.