If you change the state of something, say, liquid to solid, is it a physical or chemical change?

Which process does not require the presence of a physical substance in order to transfer heat? air in the atmosphere is heated by the ground. this warm air then rises, and cooler air falls. this is an example of what type of process? how is conduction different from radiation?

The chart shows the bid provided by four contractors to complete a job. which contractor is the most cost-effective?

What type of matter is made of only one kind of atom