cold water is denser than hot water. The same is true of most other substances as well.
One for the hot water, one for the cold.
Are you talking about pipes under a slab here? If so you probably have a hot water line under the slab leaking that is heating the water in the cold line.
HOT rises COLD sinks
cold air and water tend denser than hot air and water, so they sink
2-3 usually. 1 each for hot and cold water and possibly a third for hot water going to dishwasher.
It sinks faster in hot water than warm or cold
An egg will sink in water but will float in salt water. An egg will sink faster in hot water than it will in cold water.
depending on what type of faucet you have sometimes you need to close both hot and cold angle stop valves under sink.
Because hot water heater breaker has tripped, or hot water heater has failed.
run it under hot water in the sink
thaw it under the sink with hot water. then keep it in a bucket of hot water
They would be the stops under the sink for each of the cold and hot waters