Partial pressure

Key takeaways

Partial pressure belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Partial pressure to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Partial pressure from memory before moving on to harder problems.

Reference excerpt

In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas mixture is the sum of the partial pressures of the gases in the mixture (Dalton's Law). Partial pressure can be highlighted with a thought experiment using semipermeable pistons, as underlined by Boltzmann, Gibbs and Planck. In respiratory physiology, the partial pressure of a dissolved gas in liquid (such as oxygen in arterial blood) is also defined as the partial pressure of that gas as it would be undissolved in gas phase yet in equilibrium with the liquid. This concept is also known as blood gas tension. In this sense, the diffusion of a gas liquid is said to be driven by differences in partial pressure (not concentration). In chemistry and thermodynamics, this concept is generalized to non-ideal gases and instead called fugacity. The partial pressure of a real gas is a measure of its thermodynamic activity. Gases dissolve, diffuse, and react according to their partial pressures (which appear in reaction quotient or equilibrium constant of gas reactions) and not according to their concentrations in a gas mixture or as a solute in solution. This general property of gases is also true in chemical reactions of gases in biology.

Symbol The symbol for pressure is usually p or pp which may use a subscript to identify the pressure, and gas species are also referred to by subscript. When combined, these subscripts are applied recursively. Examples:

P 1 {\displaystyle P_{1}} or p 1 {\displaystyle p_{1}} = pressure at time 1

P H 2 {\displaystyle P_{{\ce {H2}}}} or p H 2 {\displaystyle p_{{\ce {H2}}}} = partial pressure of hydrogen

P a O 2 {\displaystyle P_{a_{{\ce {O2}}}}} or p a O 2 {\displaystyle p_{a_{{\ce {O2}}}}} or PaO2 = arterial partial pressure of oxygen

P v O 2 {\displaystyle P_{v_{{\ce {O2}}}}} or p v O 2 {\displaystyle p_{v_{{\ce {O2}}}}} or PvO2 = venous partial pressure of oxygen

Dalton's law of partial pressures

Dalton's law expresses the fact that the total pressure of a mixture of ideal gases is equal to the sum of the partial pressures of the individual gases in the mixture. This equality arises from the fact that in an ideal gas, the molecules are so far apart that they do not interact with each other. Most actual real-world gases come very close to this ideal. For example, given an ideal gas mixture of nitrogen (N2), hydrogen (H2) and ammonia (NH3):

p = p N 2 + p H 2 + p NH 3 {\displaystyle p=p_{{\ce {N2}}}+p_{{\ce {H2}}}+p_{{\ce {NH3}}}}

where:

p {\displaystyle p} = total pressure of the gas mixture

p N 2 {\displaystyle p_{{\ce {N2}}}} = partial pressure of nitrogen (N2)

p H 2 {\displaystyle p_{{\ce {H2}}}} = partial pressure of hydrogen (H2)

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Partial pressure

Start with the simplest possible case. Write down what Partial pressure claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Partial pressure before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Partial pressure ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Partial pressure

In research: Partial pressure appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Partial pressure in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Partial pressure is common in secondary-school and first-year university syllabi. It links to neighbouring topics Distillation, Engineering thermodynamics, Equilibrium chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Partial pressure outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Partial pressure in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Partial pressure means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Partial pressure out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Partial pressure in simple terms?

In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas mixture is the sum of the partial pressures of the…

Why does Partial pressure matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Partial pressure?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Partial pressure.