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Ideal gas


If we open an old chest of drawers, the metal fitting feels colder than the wood – how can that be when both materials are exposed to the same room temperature?

Metal dissipates the heat from our skin faster than wood. As a result, the temperature of our skin surface drops and we feel cold. Temperature is a physical quantity that plays an important role in thermodynamic processes: when two bodies touch each other at different temperatures, kinetic energy flows from the warmer body to the colder one. This happens until both bodies have the same temperature. The temperature is the average kinetic energy, i.e. how violently the particles move.

The best-known international units according to the SI (the International System of Units, from the French Système international d’unités) are Kelvin (K) and degrees Celsius (°C). The two units have the same scale, but a different zero point. In English-speaking countries, degrees Fahrenheit (°F) are also used.

Most physical and chemical material properties are temperature-dependent, e.g. density or viscosity. Temperature therefore plays an important role in measurement technology.

Density (mass density)

Which is heavier – a kilogram of iron or a kilogram of feathers? Children will often fall for this riddle. Of course, one kilogram remains one kilogram, but the volume differs considerably – the materials have different densities.

The mass density (ρ) describes the quotient of the mass (m) of a body and its volume (V) and is usually expressed in grams per cubic centimetre (g/cm³) or kilogram per cubic metre (kg/m³). For liquids, it is expressed in kilograms per litre (kg/l).

ρ = —–

The material determines the density, which, as the so-called intensive size, is independent of the shape and size of the body. In general, materials expand as the temperature rises, reducing their density. An exception are substances with a density anomaly such as water – at 4 °C it has its smallest volume and thus its greatest density. If the temperature drops further and the water freezes to ice, the density decreases again.


The term viscosity goes back to mistletoe – viscum. Its juice was used to make the very sticky bird glue with which the Romans used to coat their rods to catch birds.

If no further information is given, viscosity refers to shear viscosity – it describes how viscous a liquid or gas is. The higher the viscosity, the thicker the fluid. Honey or syrup are therefore highly viscous.

The term describes in detail how strong the internal friction between the particles of a substance is and how tightly they are thus bound together. On the one hand, the forces of attraction – the cohesion – between the particles are responsible for this. In the case of thinner materials, the impulse flow in the fluid also plays a role.

In addition to the term viscosity, the loss factor, storage modulus and loss modulus are also sometimes referred to.

The unit commonly used in practice is millipascal second (mPa s). The SI unit for viscosity is 1 Ns/m2 – this means that with a plate size of 1 m2 and a distance of 1 m, a force of 1 N is required to move the plates against each other at a speed of 1 m/s.


Spaghetti fun on Mount Everest? Not a good idea: due to the low air pressure, the water boils at 72°C and is not enough to cook the pasta.

Pressure accompanies us everywhere in everyday life, from the tire pressure in our cars to the headaches we get when the weather changes, caused by the change in air pressure. We also owe the fact that we can hear anything at all to sound pressure.

When a force acts perpendicularly on a certain surface, physics speaks of pressure. It does not occur in liquids and gases alone; solid bodies can also exert pressure on other bodies. According to Pascal’s principle, the pressure spreads on all sides.

The SI unit for pressure is the pascal (Pa), named after the French scientist Blaise Pascal. 1 pascal corresponds to the force of 1 newton per square metre. The unit bar is also often used: 1 bar corresponds to the pressure of a water column of 10 m in height, namely 100,000 Pa.

Ideal gas

The term “gas” is said to trace back to the Greek word “chaos” and originally specifically described the haze on water that is created by cold.

In fact, gases are somewhat chaotic, since their particles move wildly at a great distance and fill the available space evenly. Since it is often difficult to describe the “real gases” occurring in nature mathematically, science has created a simplified and idealised model. The highly simplified assumption is that the disordered particles have no expansion and only interact with each other and with the walls with hard, elastic impacts. Thanks to this idea, many thermodynamic processes of gases can be described and understood.

Some real gases, such as hydrogen and helium, are quite close to this ideal gas in their behaviour. In principle, gases increasingly move away from the behaviour of the ideal gas as the pressure increases.