Density and viscosity - it sinks through standing water as a slug instead of dispersing, and it clings to the blockage instead of running past it.

A thin liquid mixes with the water in a blocked sink and arrives diluted and spread over the whole cross-section. Very little of it contacts the blockage at working strength.

A gel formulated denser than water descends as a mass. It displaces water ahead of it and reaches the obstruction largely intact.

In a partially blocked pipe, viscosity keeps the product against the pipe wall where the deposit is, rather than draining straight through.

The trade-off is speed: a gel takes longer to reach the blockage and generally requires a longer dwell time.

Where a drain is running slowly with no standing water, a liquid or a maintenance product is usually more appropriate and cheaper.

The time on the label - typically minutes to a small number of hours - and not overnight unless the product says so.

Longer is not better. Extended contact with caustic chemistry is where damage to plastic pipework, traps and joints occurs, and where heat has time to build.

The stated dwell time is set so the chemistry has completed most of its useful work. Beyond that, little more happens.

Where the first correct application has not cleared the blockage, the answer is mechanical clearance, not a longer soak or a second dose.

Do not use a plunger or any mechanical device on a drain holding chemical - the characteristic injury in this category is caustic solution ejected upward into the face.

Flush with plenty of cold water after the dwell period, and keep the area ventilated.

Generally no - check the label, because most drain opener gels are formulated for sinks, basins, showers and floor wastes rather than for a toilet.

A toilet's trap holds a large volume of water that dilutes the product, and the blockage is usually a solid obstruction rather than a grease or hair film - which chemistry does not address.

The ceramic and the seals are also at risk from heat generated in a large standing volume.

The correct tool for a blocked toilet is a plunger or a closet auger, and neither should be used after chemical has been added.

Some products are specifically formulated and labelled for toilets; those instructions govern.

Where a toilet blocks repeatedly, the cause is usually downstream or structural, and a survey is more useful than any product.

On modern plastic and cast iron in normal use, yes - with the same caveats as any caustic product about heat, prolonged contact and aluminium.

Follow the label's material guidance. Most gels state which materials they are not suitable for.

Extended contact and the heat generated in a fully blocked pipe are the main risks to plastic components, particularly thin-walled traps and older solvent-welded joints.

Aluminium reacts with alkali and should not be exposed.

Chrome and decorative wastes mark readily from splashes on the outside of the fitting - wipe any spill immediately.

Septic tank systems deserve caution: a slug of caustic disrupts the biological process the tank depends on, and repeated use can impair it significantly. Where a property is on a septic system, biological maintenance products are the better routine choice.

Clear it mechanically - and tell whoever does so that chemical is present. Never follow with a different product.

A failed chemical attempt leaves a pipe containing caustic solution and standing water. Adding an acid product to that is one of the most dangerous things that can be done to a drain.

Mechanical clearance - a plunger only after thorough flushing, a hand auger, or professional jetting - is the correct escalation.

Anyone opening a trap or working on the pipe needs to know what is in there, and needs appropriate eye and skin protection.

Flushing with a large volume of cold water first dilutes what remains and reduces the hazard.

Repeated failures in the same drain indicate a defect rather than a soil problem, and a camera survey is the productive next step.

Upright, tightly closed, in a locked or restricted store, away from acids and hypochlorite products, and out of reach of children and animals.

Containers should stay in their original labelled packaging. Decanting caustic gel into an unlabelled bottle is both hazardous and, in a workplace, a compliance breach.

Segregation from acid products is essential, so that a leak or breakage cannot bring them into contact.

Store below any eye-level shelf. Reaching down a container of caustic gel from above head height is how facial exposure happens.

Temperature extremes affect some formulations - freezing can separate a gel and heat can thin it.

In domestic and hospitality settings, a locked cupboard is appropriate. These products cause severe injury if swallowed and the consequences of a child accessing one are serious.

No - it is a clearance product. Regular caustic dosing is hard on pipework and on the people applying it, and enzyme maintainers do the preventive job properly.

Gels are formulated to reach and attack a blockage, and their strength is unnecessary for maintaining a clear pipe.

Repeated caustic exposure ages joints and plastics, and in a septic system it disrupts the biology.

Routine handling of a corrosive product by cleaning staff is an avoidable risk when a safe alternative exists.

Enzyme and bacterial maintainers are designed for scheduled use, work on the film before it becomes a blockage, and require no protective equipment beyond gloves.

The sensible arrangement is a maintenance product on a schedule, with a gel held for the occasions when something has blocked anyway.