Toilet rim block

A toilet rim block is a block-shaped substance used in flush toilets which slowly dissolves in water. They often come in a small holder that is attached over the rim of a toilet and hangs down into the bowl, so as the toilet gets flushed, the water passes through the holder coming into contact with the block.

However, they also come loose for placement directly in-cistern (so are also usable with squat toilets), although these tend to be slightly different in composition, so as to dissolve slower, due to the constant contact with water. These may also contain a colouring, which shows up in the water (typically blue or green).

In 'liquid rims' the liquid is held in a small bottle above and connected to the holder slowly releasing the liquid into the bottom of the holder which is beneath the toilet rim, so coming into contact with the water when the toilet is flushed.

Toilet rim blocks are marketed as disinfectants and deodorizers / air fresheners, allegedly also helping to prevent the buildup of limescale in the toilet bowl.

The composition of toilet blocks can vary, but they may contain, among other components, borax (one of the ingredients of many detergents), hydroxyethylcellulose (gelling agent), troclosene sodium (disinfectant), sodium dodecylbenzenesulfonate (surfactant), sodium percarbonate (a form of oxygen bleach), sodium carbonate (washing soda) and various perfumes like e.g. limonene.

As in the closely related urinal deodorizer blocks, some of the ingredients have irritating effects when applied to skin, eyes, or when swallowed. Their ecotoxicity is rated to be harmful to aquatic organisms and may have long-term adverse effects in the aquatic environment.

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Wikipedia