Thistle Funnel, Borosilicate Glass

Quick Answer: A borosilicate thistle funnel is a long glass tube with a thistle-shaped cup at the top, made from low-expansion borosilicate glass. It is used to add liquid to a closed flask while its lower end stays under the liquid surface, most often in gas preparation set-ups that may be warmed.

The Thistle Funnel in a Gas Generator

In a typical gas preparation, a flask holding a solid reactant is closed with a two-hole bung. One hole carries the delivery tube; the other carries a thistle funnel whose tube reaches almost to the bottom of the flask. Acid is poured into the cup and runs down the tube, and as long as the tube end stays below the liquid surface, the gas produced cannot escape upwards and must leave by the delivery tube. This liquid seal is the reason for the long tube.

The funnel also acts as a simple safety valve. If the delivery tube becomes blocked, pressure in the flask pushes liquid back up the thistle tube, which is visible and warns the user before pressure can build dangerously. The bulb-shaped cup gives room for that rise and for acid to be poured in without spilling.

Borosilicate glass has low thermal expansion, so borosilicate laboratory glassware tolerates warming and temperature changes better than ordinary soda-lime glass. That matters when the generating flask is heated to start or sustain a reaction, or when hot liquid is poured in. For cold class preparations, the clear glass thistle funnel is the simpler alternative. Tube length and cup size are not stated in the listing.

Specifications

Care & Handling

• Lubricate with water or glycerol when inserting the tube into its bung, grip the stem near the rubber with a cloth wrapped round your hand, and never push on the cup.
• Keep the lower end of the tube below the liquid surface throughout the preparation.
• Heat the generating flask, not the funnel, and let the apparatus cool before dismantling it.
• Rinse the long tube with water straight after use so acid does not dry inside it.

Applications

• Gas generators in which the reaction flask is warmed, where heat resistance reduces the risk of cracking
• Adding liquid reagents to a heated flask during a reaction without opening the apparatus
• Acting as a pressure indicator in gas generation, showing a blockage as a rising liquid column
• College laboratories that standardise on borosilicate for all glassware used near heat

Why Choose LabEquip

College chemistry laboratories, and schools that prefer heat-resistant glass for any set-up near a burner, choose this version of the thistle funnel. LabEquip offers it among its chemistry lab equipment; send quantities and tube length through the contact page.

Frequently Asked Questions

Why must the end of the thistle tube be under the liquid?

If the tube end is above the liquid, gas escapes up the tube and out of the cup instead of passing through the delivery tube. Keeping it submerged forms a liquid seal, so the only way out for the gas is the delivery tube.

What is the advantage of borosilicate for a thistle funnel?

Borosilicate glass expands very little with heat, so it is far less likely to crack when the generating flask is warmed or when hot liquid is poured in. For preparations at room temperature, ordinary glass is usually adequate.

Can a thistle funnel control the rate of acid addition?

Only roughly, because the acid runs in as fast as it is poured. When a controlled, dropwise addition is needed, a dropping funnel or a separating funnel with a stopcock is used instead.

Why did liquid rise up the thistle tube during my experiment?

Pressure inside the flask increased, usually because the delivery tube is blocked, kinked or dipping too deep into water. Stop adding acid, remove any heat, and clear the delivery tube before continuing.

How do I fit a thistle funnel into a rubber bung safely?

Wet the tube and the bung hole with water or glycerol, wrap a cloth around the tube, and grip it close to the bung, twisting gently as you push. Never press on the cup, which is the weakest part.

Can the thistle funnel be used for osmosis experiments?

Yes. With a membrane tied over the mouth of the cup, the funnel inverted, filled with sugar solution and stood in water, osmosis shows as liquid rising up the tube. Schools often use the clear glass version for this.

Last Updated: September 2026