Can anyone help me with this ( no links )

An empty fuel tank can still contain and therefore can be even more dangerous than one full of liquid fuel.

What is the independent variable in this investigation? mass volume sample number substance density

Which of the following is an example of physical change not a chemical change? a) a log gives off heat and light as it burns. b) a tree stores energy from the sun in its fruit. c) a penny lost in the grass slowly changes color. d) a water pipe freezes and cracks on a cold night.