Final answer:
Birds like the black-throated loon can float due to water's surface tension, which is disrupted by soap resulting in the birds sinking.
Step-by-step explanation:
The black-throated loon, like many other birds such as ducks and geese, is able to float on the surface of the water due to the property of water known as surface tension. This surface tension is a result of the cohesion between water molecules at the water-air interface, creating a sort of 'skin' on the water's surface that can support the weight of the birds as long as their feathers are not wetted. This is similar to how insects like water striders can walk on water. The birds' feathers have special waterproofing properties that prevent them from becoming soaked, allowing them to maintain their buoyancy. However, when soap is added to water, it reduces the water's surface tension, which can no longer support the weight of the birds, causing them to sink.