Use a double stroke where one stroke is slightly wider than the one
above it in the stacking order.
Go to the Appearance panel and click the options flyout in the upper
right corner. Select "Add New Stroke". Repeat again (if you don't
already see two strokes) so you now have 2 strokes in the Appearance
panel. Select the bottom-most stroke and choose black as your stroke
color, then adjust the stroke weight from the Stroke panel so that it
is slightly wider than the one above it. Now select the top-most
stroke from within the Appearance panel and select the appropriate
color to represent water, steam, gases, etc.
Make sure and drag the result into your Graphic Styles panel for easy
access later as you draw.