By Kevin Beck | Updated Mar 24, 2022
When we think of a liquid, we often picture water in a glass, a stream in a river, or the slick surface of a pond. Yet the everyday experience of a liquid doesn’t capture the full scientific picture. Below, we delve into what makes a substance a liquid, how it behaves, and why liquids matter—from engineering to human biology.
All matter exists in one of three primary states. Solids have tightly packed, regularly arranged particles that vibrate in place. Gases have widely spaced particles that move freely and occupy any available volume. Liquids sit between these extremes: their particles are close together but lack a fixed shape, allowing them to flow and conform to their containers.
In physics, a fluid refers to any substance that cannot resist deformation. This umbrella term includes both liquids and gases. Fluids can be described by the same fundamental equations—most notably the Navier‑Stokes equations—whether the substance is water or air. This unified treatment explains why a marathon runner must manage fluid loss just as carefully as an aircraft pilot manages airflow.
Fluids are characterized by three broad categories of properties:
These properties govern everything from how a drop of oil spreads on a surface to how air flows around an airplane wing.
Water and air dominate everyday discussions of fluids, but a variety of other liquids—oil, gasoline, kerosene, solvents, and even beverages—play critical roles in industry and daily convenience. Many of these liquids are hazardous; proper storage is essential to prevent accidental ingestion or exposure.
In the human body, liquids are essential. Although blood contains solids (cells and proteins), its plasma component behaves as a liquid. Proper hydration is vital for athletic performance, yet many athletes still suffer dehydration despite frequent refueling.
Fluid mechanics studies how fluids move and interact with their environment. Unlike solids, fluids can shear—layers of fluid slide past one another—creating phenomena such as eddies and turbulence. The shear stress τ is calculated as:
τ = μ(du/dy)
where μ is dynamic viscosity and du/dy is the velocity gradient.
Two critical forces in aerodynamics and hydrodynamics are drag and lift:
Here, ρ is fluid density, A is cross‑sectional area, v is velocity, and C_D or C_L are shape‑dependent constants.
Water constitutes roughly 60 % of an adult’s body weight. Two‑thirds of that—about 40 % of total body weight—is intracellular fluid; the remaining third is extracellular fluid. Blood plasma, which is the liquid part of blood, accounts for about one‑quarter of the extracellular fluid, or 5 % of total body weight.
For a 70‑kg (154‑lb) individual:
These calculations illustrate the importance of maintaining fluid balance for health and performance.
