Tube, Ignition

Quick Answer: An ignition tube is a small, usually thick-walled glass test tube used for heating tiny amounts of solid strongly in a flame. In qualitative analysis it is the tube for the sodium fusion (Lassaigne’s) test and for dry heating tests on salts.

A Small Tube for Fierce Heating

Some tests call for heating a few milligrams of solid until it glows, which would soften or crack a thin test tube. The ignition tube is short and narrow, usually with a thicker wall, so a small sample can be heated directly in a Bunsen flame. Being small, it heats quickly, uses very little material and is normally treated as a consumable.

Its most familiar job is Lassaigne’s sodium fusion test for nitrogen, sulfur and halogens in organic compounds. A small piece of sodium and a little of the compound are heated together in the tube until red hot, and the hot tube is plunged into distilled water in a china dish, where it shatters. The solution, filtered from glass and charcoal, is then tested with the usual reagents for cyanide, sulfide and halide ions.

In inorganic salt analysis the same tubes are used for dry heating tests. A salt heated alone may give off water vapour, oxygen, carbon dioxide or coloured gases, or change colour, and these observations point towards the ions present. The listing does not state tube size or glass type.

Specifications

Applications

• Lassaigne’s sodium fusion test for nitrogen, sulfur and halogens in organic compounds
• Dry heating tests on inorganic salts during salt analysis practicals
• Observing sublimation of ammonium chloride or iodine in a small, easily handled tube

Care & Handling

• Wear eye protection and use a safety screen for sodium fusion; the tube is meant to break in water and fragments can fly.
• Warm the tube gradually before heating it strongly, so it does not crack from sudden expansion.
• Discard used and broken tubes in a glass-waste container, since fused residues rarely clean out.
• Store sodium metal under paraffin oil and cut only small pieces with a dry knife.

Why Choose LabEquip

Ignition tubes are used up quickly in school and college qualitative analysis, so laboratories order them in bulk before practical sessions. LabEquip lists them in the chemistry lab equipment range next to the larger combustion tube for strong heating in a gas stream; send quantities through the contact page.

Frequently Asked Questions

What is the difference between an ignition tube and a test tube?

An ignition tube is smaller and usually thicker-walled, made for heating a tiny sample strongly in a flame. A test tube is larger and used for reactions in solution and moderate heating.

Why is the tube broken in Lassaigne’s test?

After fusion, the sodium compounds formed are inside a red-hot tube. Plunging it into distilled water shatters the glass so the fused mass dissolves, giving the sodium fusion extract that is filtered and tested.

Is the sodium fusion test safe for students?

It is carried out under close supervision, with eye protection and a safety screen, because hot sodium reacts vigorously with water. Only a small piece of freshly cut sodium is used, and the tube is plunged into water in a china dish.

What can dry heating of a salt show?

Water condensing on the cooler part of the tube, a colour change in the residue, or gases such as oxygen, carbon dioxide or brown nitrogen dioxide suggest particular ions. They guide which confirmatory tests to do next.

Can the tubes be reused?

Tubes used for sodium fusion are destroyed in the test. Tubes used for gentle heating can sometimes be cleaned and reused, but any with fused residue or cracks should be discarded.

Why does my sodium fusion extract give no result?

The compound may not have been heated long enough to decompose fully, too little sodium may have been used, or the extract was not boiled and filtered before testing. Heating the tube at red heat for a minute or two usually gives a usable extract.

Last Updated: September 2026