Pneumatic Trough

Quick Answer: A pneumatic trough is a wide, shallow water-filled vessel used to collect gases by displacement of water. A gas jar or test tube is filled with water, inverted in the trough, and gas from a delivery tube bubbles up into it, pushing the water out.

Collecting a Gas Over Water

Collection over water is the simplest way to gather a gas that does not dissolve much in water. The trough is filled with water, and a gas jar is filled to the brim, covered with a glass plate, inverted and stood mouth-down in the water. Many troughs have a raised shelf, sometimes called a beehive shelf, with an opening through which the delivery tube enters, so the jar can stand steadily over the gas stream; this listing does not say whether a shelf is included.

The method works well for hydrogen, oxygen and nitrogen, and reasonably for carbon dioxide, which dissolves a little. It is unsuitable for very soluble gases such as ammonia, hydrogen chloride or sulfur dioxide, which dissolve before they can collect. A useful bonus is that the first jar, which contains air pushed out of the apparatus, can be discarded before pure gas is collected, and the falling water level shows how much gas has been gathered.

The trough material, shape and size are not stated in the listing. Rubber tubing and delivery tubes of suitable length link it to the generating flask or heated test tube.

Specifications

Care & Handling

• When a gas is prepared by heating, raise the end of the delivery tube clear of the water before taking the flame away, so cold water cannot be sucked back into the hot tube.
• Fill the trough only to a level that will not overflow when jars are lowered into it.
• Empty and dry the trough after use to prevent algae growth and scale.
• Stand the trough on a tray or a dry area of bench away from electrical sockets.

Applications

• Collecting the oxygen released when hydrogen peroxide decomposes over a manganese(IV) oxide catalyst, then relighting a glowing splint with it
• Collecting hydrogen from a metal and dilute acid for the lighted splint test
• Following the rate of a reaction by collecting gas in an inverted measuring cylinder
• Collecting oxygen from heated potassium permanganate in a test tube, a traditional school preparation

Why Choose LabEquip

Schools teaching gas preparation and tests for gases usually need one trough per working group. LabEquip stocks the trough among its chemistry lab equipment; send quantities for your class through the contact page.

Frequently Asked Questions

Which gases can be collected in a pneumatic trough?

Gases that are only slightly soluble in water, such as hydrogen, oxygen and nitrogen, collect well. Carbon dioxide can be collected with some loss. Very soluble gases, including ammonia and hydrogen chloride, cannot be collected over water.

What is suck-back and how do I prevent it?

When a heated tube cools, the gas inside contracts and cold water can be drawn back up the delivery tube into the hot glass, cracking it. Always take the delivery tube out of the water before stopping the heating.

Why should the first jar of gas be discarded?

The first gas to come through is mostly air pushed out of the flask and tubing. Discarding it, or letting the gas bubble away for a short time first, gives a purer sample for tests.

How do I invert a gas jar without letting air in?

Fill the jar completely with water, slide a greased glass plate over the mouth, turn it over with the plate held in place, lower the mouth under the water, and remove the plate underwater.

What is a beehive shelf?

It is a small raised stand with a slot in the side for the delivery tube and a hole in the top. An inverted gas jar rests on it over the hole, which holds the jar steady and lets several jars be filled in turn.

Can the trough be used to measure the volume of gas produced?

Yes. Collecting gas in an inverted measuring cylinder full of water lets you read the volume directly, which is a common way of following the rate of a reaction such as magnesium with dilute acid.

Last Updated: September 2026