The question of how a snowflake comes into existence within a cloud touches on the fundamental differences between two types of change in matter. Many natural processes, especially those involving water, can appear complex enough to be a chemical transformation. However, the formation of a snowflake in the atmosphere is definitively a physical change. To understand why, it is first necessary to examine the definitions used to categorize how matter changes.
The Criteria of Chemical Change
A chemical change is a transformation that results in the creation of an entirely new substance with a different chemical identity. This fundamental alteration occurs at the molecular level through the breaking of existing chemical bonds and the formation of new ones. The atoms that make up the starting materials, known as reactants, are rearranged to produce different substances called products.
For example, when wood burns, the carbon compounds in the wood react with oxygen in the air, breaking those bonds and forming new bonds to create carbon dioxide and water vapor. This molecular rearrangement is accompanied by a significant release or absorption of energy. The resulting products possess chemical properties that are wholly distinct from the original reactants, confirming that a chemical transformation has taken place.
The Nature of Physical Change
In contrast to a chemical transformation, a physical change leaves the chemical composition of a substance unaltered. While the appearance, shape, or state of matter may be dramatically different, the molecular structure itself does not change. The forces affected in a physical change are intermolecular forces, which are weaker attractions between molecules, rather than the stronger bonds within them.
The most common examples of physical change involve phase transitions, where a substance shifts between its solid, liquid, or gaseous states. Processes like melting wax, boiling water, or crushing a rock are all physical changes because the material’s chemical formula remains constant. These changes are often reversible simply by adjusting external conditions, such as temperature or pressure. Phase changes occur when energy is added or removed, affecting the physical arrangement of molecules but not their internal chemical bonds.
Applying the Concepts to Snowflake Formation
Snowflake formation is a physical phase transition that occurs in the cold, moist environment of a cloud. The process involves water molecules transitioning directly from a gaseous state (water vapor) to a solid state (ice) without first becoming a liquid. This specific transition is known as deposition, and it requires the water vapor to lose heat energy to the surrounding air.
The process begins when water vapor molecules adhere to a tiny atmospheric particle, such as dust or pollen, which acts as a nucleation site. As the vapor loses energy, the individual H₂O molecules slow down and begin to arrange themselves into a highly ordered, six-sided crystalline lattice. The water molecules maintain their structure of two hydrogen atoms bonded to one oxygen atom; no bonds are broken or formed, and no new chemical compound is created.
Alternatively, snowflakes can grow through a process called accretion, where the nascent ice crystal collides with supercooled water droplets. Supercooled water is liquid water that exists at temperatures below its normal freezing point because it lacks a surface to crystallize upon. Upon contact with the ice crystal, these liquid droplets instantly freeze, further contributing to the snowflake’s growth. Whether by direct deposition or by freezing supercooled liquid, the chemical substance remains H₂O throughout, confirming the process is a physical change.
The intricate shapes of snowflakes are determined solely by the temperature and humidity conditions the crystal encounters as it falls. These environmental factors influence the rate at which additional water molecules attach, causing the crystal to grow in varying patterns. The entire mechanism is about energy exchange and the resultant physical arrangement of water molecules, which aligns with the definition of a physical change.