A scientific model is a conceptual, mathematical or physical representation of a real-world phenomenon. A model is generally constructed for an object or process when it is at least partially understood, but difficult to observe directly. Examples include sticks and balls representing molecules, mathematical models of planetary movements or conceptual principles like the ideal gas law. Because of the infinite variations actually found in nature, all but the simplest and most vague models are imperfect representations of real-world phenomena.
The ideal gas law is an excellent example of a model, in terms of both their usefulness and imperfections. It states that, within any particular volume of a gas at a particular temperature and pressure, the number of gas molecules or atoms can be exactly calculated, regardless of the actual composition of the gas involved. However, it is called the ideal gas law for a reason. It assumes uniform conditions throughout the given volume of gas, a condition that can almost never actually be met. Bodies of gases in the real world, even within sealed containers, have variations in temperature and pressure, along with accompanying currents. Similarly, the equation for the volume of a sphere gives a good approximation for the volume of roughly spherical objects, but no perfect spheres are likely to exist in actual nature.