Question

A sample of dry ice (solid \(\mathrm{CO}_{2}\) ) starts to form a white cloud upon standing. Gaseous carbon dioxide is formed. Is this change a chemical reaction? Explain your answer.

Short answer

No, this change is not a chemical reaction. The process is sublimation, a type of phase transition, which is a physical change, not a chemical reaction.

Step-by-step solution

Understand what a chemical reaction is

A chemical reaction involves a process that leads to transformation of one set of chemical substances to another. This is characterized by breaking of bonds in reactants and forming of new bonds in products, resulting in a change in the molecular or ionic structure of a substance.

Identify the process described in the problem

The given scenario describes that the dry ice (solid CO2) forms a white cloud of gaseous carbon dioxide upon standing. This process is known as sublimation, where a solid substance changes directly into a gas without first becoming a liquid.

Conclude if the process is a chemical reaction or not

Sublimation is not a chemical reaction, but a physical change. The molecules of carbon dioxide remain as CO2 molecules during sublimation and do not undergo any change in the molecular structure, and thus no new substance is formed. Therefore, it is not a chemical reaction.

Key concepts

Sublimation

Have you ever seen a block of dry ice create a misty fog without ever turning into a liquid? This fascinating transformation is known as sublimation, a process during which a substance changes directly from a solid to a gas without passing through the intermediate liquid phase.

Sublimation occurs under specific conditions of temperature and pressure, typically at low pressure or at temperatures below the substance's triple point in its phase diagram. Think of it as a special kind of 'magic trick' that certain compounds, like solid carbon dioxide or 'dry ice', can perform under the right conditions.

Understanding the Sublimation of Dry Ice

When dry ice is exposed to air, it absorbs heat and sublimes, directly transforming into carbon dioxide gas and creating that characteristic white cloud. This change is purely physical; the molecular structure of the compound remains unchanged. Solid CO2 goes from a rigid, structured solid straight to a free-moving gas - an example of sublimation you might witness in a science class or even in theatrical productions for special smoke effects.

Physical Change vs Chemical Change

When a student encounters the mesmerizing act of sublimation or any other change in a substance, there's often confusion between physical and chemical changes. It's essential to distinguish the two to properly understand the nature of the change happening before your eyes.

A physical change involves alterations in the state of matter (solid, liquid, gas) or the appearance of a substance, without changing its molecular composition. Melting, freezing, and sublimation are classic examples of physical changes where the substance retains its identity.

Chemical Changes: A Transformation

On the other hand, a chemical change results in the formation of one or more new substances. This involves making and breaking chemical bonds, which leads to new molecular structures. Burning wood, rusting iron, and cooking an egg are all chemical changes because they produce substances with entirely new chemical properties.

As for the case of dry ice subliming into carbon dioxide gas, it's a physical change—a re-arrangement of how molecules are packed, not a transformation into a new chemical identity.

Molecular Structure

Digging deeper into physical and chemical changes, it's the molecular structure that's the real star of the show. The molecular structure refers to the three-dimensional arrangement of atoms within a molecule, held together by chemical bonds.

An understanding of molecular structure is vital because it determines the properties and behavior of the substance. For instance, the reason why water turns to ice at low temperatures and steam at high temperatures relates to the movement and arrangement of its H2O molecules.

Stability in Sublimation

In sublimation, like the case of dry ice becoming gas, there is no change in the molecular structure of carbon dioxide. The molecules maintain their form and simply move from being close together (solid) to spreading out far apart (gas). Recognizing that their bonding stays constant is the key to understanding why sublimation is a physical change, not a chemical one.