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The accepted answer in When is hand sanitizer appropriate/sufficient in place of warm water and soap? mentions that you should avoid bar soap yet the link provided no longer says this.
I have heard before that it is advised to use bottled soft soap rather than bar/block soap as soft soap is more hygienic.
Is this idea backed by science?
TLDR: They're both equally effective. Technically, liquid soap is "cleaner" than bar soap unless the soap container is usually refilled.
In more detail:
From my understanding, any soap is effective at destroying viruses because it literally shreds apart the coating of the virus, obliterating it in the process. This is due to the nature of the soap being both hydrophilic (bonding to water) and lipidphilic (bonding to the viruses' lipid shells). So you would be seeing the same outcome from both bar and liquid soap.
Then, the question remains on what happens to the shredded bodies of the pathogens. Well, when you rinse with water, any of the viral remains will wash down the drain, so again there is effectively no argument for neither bar nor liquid soap.
However, when first making contact with bar soap, you are effectively transferring some of the pathogens onto the surface of the soap. These pathogens will still be destroyed, but their remains will sit on the soap. So in that sense, using bar soap is "less clean" because you are leaving viral debris on the surface of the soap (though this debris is benign). Score 1 for liquid soap
Now, if the liquid soap is being refilled regularly (like a refillable pump bottle, or a refillable public bathroom dispensor) then this soap container is likely to already be contaminated with live pathogens. Score 1 for bar soap
In the end, both bar soap and liquid soap are equally effective at destroying pathogens. If you wanted to get really nitty-gritty, I'd say liquid soap is the "cleanest" option, unless the container is one which gets refilled, then the bar soap is the "cleanest".
