Question

During a very cold winter, the temperature may remain below freezing for extended periods. However, fallen snow can still disappear, even though it cannot melt. This is possible because a solid can vaporize directly, without passing through the liquid state. Is this process (sublimation) a physical or a chemical change?

Short answer

Sublimation is a physical change because it involves a change in the state of matter, from solid to gas, without altering the fundamental chemical structure or composition of the substance. No new substances are formed during sublimation, and the substance's identity remains the same throughout the process.

Step-by-step solution

Define Physical Change

A physical change is a change that occurs without altering the fundamental chemical structure or composition of a substance. It usually affects the form of a substance, such as its shape, size, or state of matter (i.e., solid, liquid, or gas), but the substance's identity remains the same.

Define Chemical Change

A chemical change, also known as a chemical reaction, is a process that results in the formation of new substances with different properties than the original substance(s). This occurs due to the breaking and forming of chemical bonds between atoms, which leads to a change in the substance's molecular structure and composition.

Understand Sublimation

Sublimation is the process by which a solid substance transitions directly into the gaseous state, without going through the intermediate liquid state. The process of sublimation occurs when the molecules in the solid state overcome the attractive forces between them and enter the gaseous state. Common examples of sublimation include the vaporization of dry ice (solid carbon dioxide) or the disappearance of snow or ice in cold weather.

Determine the Type of Change

In the process of sublimation, the substance in the solid state undergoes a change in its state of matter, becoming a gas. However, no new substance is formed, and the fundamental chemical structure and composition of the substance remain the same. Thus, sublimation is a physical change, not a chemical change.

Conclusion

The process of sublimation, in which a solid vaporizes directly into a gas without passing through the liquid state, is a physical change. This is because the substance's fundamental chemical structure and composition do not change during sublimation, and no new substance is formed.

Key concepts

Physical Change

A physical change involves a transformation in the physical properties of a substance without altering its chemical composition. During this type of change, you might notice modifications in the substance's appearance, such as a change in shape, size, or phase (solid, liquid, or gas). However, the substance itself remains the same at the molecular level.

Examples of physical changes include:

• Melting ice into water
• Boiling water into steam
• Breaking a glass
• Sublimation of ice and dry ice

In each of these examples, the fundamental structure of the molecules remains unchanged. The ability to reverse the process often characterizes physical changes. For instance, water can be frozen to form ice again. In the context of sublimation, this is clearly a physical change because snow or ice becoming vapor does not involve a change in what the substance ultimately is: water (H₂O).

Chemical Change

A chemical change, on the other hand, occurs when there is a transformation in the chemical makeup of the substances involved. These changes involve making or breaking chemical bonds, resulting in new substances with properties different from those of the starting materials.

Indicators of chemical changes include:

• Color change
• Formation of a precipitate
• Emission or absorption of heat (exothermic or endothermic reactions)
• Production of gas (bubbles or fumes)

During a chemical change, the original substances lose their individual properties, and new materials are formed. An example would be the rusting of iron, where iron reacts with oxygen to form a new substance, iron oxide. This process is irreversible and signifies a true change in the material's chemical structure. With sublimation, no such changes or formations of new products occur, differentiating it from a chemical change.

State of Matter Transition

A state of matter transition refers to the change between solid, liquid, and gaseous states of a substance. This transition is often influenced by changes in temperature and pressure. Sublimation is a specific type of state of matter transition where a solid spontaneously turns into a gas, bypassing the liquid phase.

Some key aspects of sublimation include:

• Occurs under certain environmental conditions, often low pressure and sufficient heat
• Does not alter the molecular structure of the substance
• Well-known examples include dry ice (solid CO₂) sublimating into carbon dioxide gas
• Snow disappearing in freezing temperatures

The sublimation process provides a clear illustration of a physical change through a state of matter transition. Understanding these transitions helps in comprehending how substances interact with different environmental conditions without chemically changing. This is why sublimate processes like snow turning into vapor do not qualify as chemical changes but rather as fascinating examples of physical state transitions.