---
title: "Constant-Pressure Calorimeter | Intro to Chemistry"
description: "A constant-pressure calorimeter measures heat changes at atmospheric pressure in Intro to Chemistry, letting you calculate heat of reaction or solution."
canonical: "https://fiveable.me/intro-chem/key-terms/constant-pressure-calorimeter"
type: "key-term"
subject: "Intro to Chemistry"
unit: "Unit 5"
---

# Constant-Pressure Calorimeter | Intro to Chemistry

## Definition

A constant-pressure calorimeter is a device that measures heat transfer while the pressure stays constant, usually at atmospheric pressure. In Intro to Chemistry, it is used to find heats of reaction, solution, and specific heat from temperature change.

## What It Is

A constant-pressure calorimeter is a chemistry lab device that measures heat transfer under constant pressure, usually the same pressure as the air around it. In Intro to Chemistry, this is the setup you use when you want to track how much heat a reaction gives off or absorbs without sealing the system tightly.

The classic example is a coffee-cup style calorimeter: an insulated container, often a foam cup, holds a known mass of water or solution, and you stir and measure the temperature change as the reaction happens. The pressure stays essentially constant because the container is open to the atmosphere. That matters because the heat you measure in this setup is the heat exchanged at constant pressure, which chemists connect to enthalpy changes.

The basic idea behind the measurement is energy conservation. If the reaction releases heat, the surroundings in the calorimeter warm up. If the reaction absorbs heat, the surroundings cool down. You record the temperature change, use the mass of the solution and its specific heat capacity, and calculate the heat gained or lost by the surroundings. Then you flip the sign to get the heat of the reaction.

A common form of the equation is q = m c ΔT, where q is heat, m is mass, c is specific heat capacity, and ΔT is the temperature change. In a simple aqueous solution, the solution is often treated like water unless the problem says otherwise. The calorimeter itself may also absorb some heat, which is why some lab problems include a calorimeter constant.

Constant-pressure calorimetry shows up any time the chemistry happens in a beaker, cup, or open container. Neutralization reactions, dissolving salts, and some phase change measurements are typical examples. The setup is less rigid than a bomb-style device, but that is exactly why it fits Intro to Chemistry labs well: you can measure temperature changes with simple equipment and connect them directly to thermodynamics.

## Why It Matters

This term matters because it is one of the main ways Intro to Chemistry turns a temperature change into a heat calculation. You are not just watching a solution get warmer or cooler, you are translating that change into a quantitative energy value.

That skill sits right in the middle of calorimetry, thermochemistry, and stoichiometry. If you know the mass, specific heat capacity, and temperature change, you can find the heat absorbed by the solution. From there, you can infer the heat released or absorbed by the reaction itself, which is a common lab calculation.

It also helps you separate what the reaction is doing from what the surroundings are doing. A lot of students first say the reaction temperature changed, but in calorimetry the measured temperature change belongs to the solution or water in the calorimeter. The reaction and the surroundings exchange energy, and the calorimeter is the tool that makes that exchange measurable.

You will also see this idea when comparing different calorimeters. Constant-pressure calorimeters are used for reactions in open containers, while constant-volume calorimeters are built differently and measure heat under fixed volume instead. Knowing which setup you are looking at tells you which thermodynamic quantity you are finding and which equation applies.

## Connections

### Calorimetry

Constant-pressure calorimetry is one type of calorimetry, so the bigger idea is the same: measure heat by tracking temperature change. When you see a lab question about heat flow, the first step is often to decide what object is warming or cooling and what equation fits the setup. This term is the specific open-container version of that process.

### Heat Capacity

Heat capacity is part of the calculation because the calorimeter contents absorb heat according to how much substance you have and what its heat capacity is. If a problem gives you a sample and asks for heat transfer, you usually need mass and specific heat capacity before you can solve it. Constant-pressure calorimeter problems often depend on that formula.

### Heat of Reaction

The point of many constant-pressure calorimeter measurements is to find the heat of reaction. You measure the temperature change of the surroundings, calculate q for the solution, and then use the opposite sign for the reaction. That gives you a reaction enthalpy in a simple lab context.

### [Coffee-Cup Calorimeter](/intro-chem/key-terms/coffee-cup-calorimeter)

A coffee-cup calorimeter is the most common example of a constant-pressure calorimeter in Intro to Chemistry. It is usually a foam cup with a lid and thermometer, and it is open to atmospheric pressure. If you see a drawing or lab photo, this is often the setup being tested.

## On the AP Exam

A quiz question might give you a temperature change from a coffee-cup calorimeter and ask for the heat of reaction. Your job is to identify the system, decide that the solution is the surroundings, and use q = mcΔT for the liquid in the cup. Then you reverse the sign to get the reaction's heat.

Lab questions also ask you to explain why the pressure is considered constant or why that matters. A strong answer says the calorimeter is open to the atmosphere, so the pressure stays near atmospheric pressure during the reaction. If the problem includes a calorimeter constant, you would add the heat absorbed by the calorimeter itself instead of treating the cup as perfectly insulated.

## Constant-Pressure Calorimeter vs Constant-Volume Calorimeter

These two are easy to mix up because both measure heat through temperature change, but the setup is different. A constant-pressure calorimeter is open to the atmosphere and is usually a coffee-cup style device. A constant-volume calorimeter is sealed, so it is used for reactions where the volume cannot change, and it measures a different heat condition.

## Key Takeaways

- A constant-pressure calorimeter measures heat change while the pressure stays essentially constant, usually at atmospheric pressure.
- In Intro to Chemistry, the most common version is a coffee-cup calorimeter used for reactions in solution.
- The temperature change of the solution tells you how much heat was absorbed or released, usually through q = mcΔT.
- The sign of q for the surroundings and the sign of the reaction are opposite.
- If the calorimeter is not perfectly ideal, a calorimeter constant may be needed to account for heat absorbed by the container itself.

## FAQs

### What is a constant-pressure calorimeter in Intro to Chemistry?

It is a device used to measure heat transfer while the pressure stays constant, usually at atmospheric pressure. In Intro to Chemistry, it is often a foam-cup or coffee-cup setup used to study reactions in solution. You use the temperature change to calculate heat gained or lost.

### How does a constant-pressure calorimeter work?

A reaction happens in a container with a known amount of water or solution, and you measure the temperature change. If the temperature rises, the reaction released heat; if it falls, the reaction absorbed heat. That temperature change becomes a heat calculation with q = mcΔT.

### Is a coffee-cup calorimeter the same as a constant-pressure calorimeter?

Yes, in Intro to Chemistry, a coffee-cup calorimeter is the standard example of a constant-pressure calorimeter. It is open to the atmosphere, so the pressure stays nearly constant during the reaction. That makes it useful for solution reactions and heat of reaction problems.

### What do you calculate with a constant-pressure calorimeter?

You usually calculate heat of reaction, heat of solution, or the heat absorbed by a substance. The measured temperature change lets you find the heat of the surroundings first, then infer the heat change of the chemical process. Some lab problems also ask for specific heat capacity.

## Related Study Guides

- [5.2 Calorimetry](/intro-chem/unit-5/2-calorimetry/study-guide/uc0nUeho8xuwdEYf)

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