Mixing cornstarch and water creates Oobleck, a unique substance that flows like a liquid but resists force like a solid. This behavior raises a fundamental question: does the creation of Oobleck represent a permanent chemical alteration or a temporary physical rearrangement? Determining the nature of this change requires understanding the scientific differences between these two types of transformations.
Distinguishing Chemical vs. Physical Changes
A physical change alters a substance’s form or appearance without changing its fundamental chemical identity. Examples include changes in state, such as melting or freezing, or changes in size, like grinding a powder. These transformations are generally reversible, and no new chemical compound is formed.
In contrast, a chemical change, or chemical reaction, results in the formation of one or more entirely new substances with different properties from the starting materials. This transformation involves the breaking and forming of chemical bonds between atoms. Observable signs often include the production of a gas, a spontaneous change in temperature, or an irreversible shift in color.
The Simple Composition of Oobleck
Oobleck is made by combining cornstarch, a carbohydrate polymer, with water. The resulting mixture is a heterogeneous suspension, where tiny solid particles are dispersed throughout a liquid. The cornstarch granules are not dissolved, meaning they retain their original chemical structure.
The water molecules and the cornstarch molecules remain chemically separate and intact. If the Oobleck is left undisturbed, the heavier cornstarch particles will eventually settle out of the liquid. This separation demonstrates that the components have not bonded together to form a new compound.
The Classification: A Physical Change
The act of mixing cornstarch and water to create Oobleck is classified as a physical change. The formation of this mixture does not produce any new chemical substances, as the cornstarch and water molecules retain their individual chemical identities. No chemical bonds are broken or formed between the two ingredients during the mixing process.
The change is also considered physically reversible, which is a hallmark of a physical change. If the water is allowed to evaporate, the dry cornstarch powder will be left behind, demonstrating that the original components can be recovered. The transformation only affects the physical arrangement of the particles, not their underlying molecular composition.
The Unique Behavior of Non-Newtonian Fluids
While its creation is a physical change, Oobleck’s unique physical properties make it scientifically interesting. The mixture belongs to a class of materials known as non-Newtonian fluids because its viscosity, or resistance to flow, is not constant. Unlike a Newtonian fluid, Oobleck’s viscosity changes depending on the amount of stress or force applied to it.
Oobleck exhibits shear-thickening behavior, meaning it becomes more viscous and solid-like when a rapid, strong force is applied. When force is applied slowly, the water acts as a lubricant, allowing the cornstarch particles to slide past one another, and the mixture flows like a liquid.
A sudden impact pushes the cornstarch particles closer together, forcing them to temporarily jam or lock up due to friction. This rapid increase in particle-to-particle contact creates a transient, rigid, solid-like structure that resists the applied force. The mixture instantly reverts back to its liquid-like state once the high stress is removed. This behavior is a function of the physical interaction and arrangement of the particles, not a chemical reaction.