Closely spaced contour lines represent steep slopes, conversely, contour lines that are spaced far apart represent gentle slopes. Contour lines trend up valleys and form a "V" or a "U" where they cross a stream.
That would be propane
Propane is neither an acid nor base, tis' neutral
Solids maintain their shape, whereas fluids do not because the molecules in solids maintain a regular pattern and only vibrate, or move very slowly