where do the solids go?

Comments on Cecil's Columns/Staff Reports
#1

the answer re candle wax does not explain where the solids in the candle wax go.
Put differently: weigh 100 sq.ft. by 12 in of soil = weight A
Plant a tree and let it grow to maturity. Dig up the tree and burn it, roots and all. What is left is the inert material, weight B
Query: re-weigh A - is that weight now A minus B?
If yes - how is it that we can plant crops for years on end without the fields sinking?
(weight B)
If no - where does weight B come from?
Have a headache yet? :slight_smile:

LINK TO COLUMN: http://www.straightdope.com/columns/read/812/where-does-the-candle-wax-go

#2

Candle wax is basically methane (carbon and hydrogen) but in a stretched-out form; when burned, it reacts with oxygen and the byproducts are carbon dioxide and water (ideally, in practice you will also get carbon monoxide, soot and other hydrocarbons):

CH4 + 2 O2 = CO2 + 2 H2O

(replace CH4 with a longer chain form like C2H6, etc; these molecules are termed alkanes and have the formula CnH2n+2)

For your other question, trees convert carbon dioxide from the air and water from rain into glucose, which in turn is polymerized into cellulose. There are also minor nutrients that are taken from the soil, and in fact the soil will become depleted in these nutrients if you keep growing crops in the same area year after year without adding them back or adding fertilizer, but there won’t be much of an effect on the weight (not including soil erosion).

#3

The solid candle becomes non-solid.

Are there solids in an ice cube? Where do they go when the ice melts?

If you’re referring to ‘solid’ as something other then solid wax, please define the term and prove that such matter remains solid under all physical conditions. :slight_smile:

#4

Yes. Why use 100 cubic feet of soil, a full grown tree, and a decade or two?

Why not an eight inch pot, a pothos, and a year, maximum? Then you could even do several experiments, instead of just one.

It’s been done. The weight of the soil does decrease, but very little. The additional material in plant is from carbon dioxide in the air and water.

#5

Right. Most of the mass of the tree comes from air and water. Specifically, by photosynthesis the plant takes carbon dioxide and water and sunlight, and turns that into sugar. The sugar is then chained together to form cellulose. So a tree’s mass comes literally from the air. There are some minerals, equivalent in weight to the small amount of ash left over when wood is completely burned. But most of the tree turns back into carbon dioxide and water when combined with oxygen.

Same thing with a candle. The CH3-CH2-CH2-CH2…CH2-CH2-CH3 chains in the wax or parrafin combine with lots of O2 to form CO2 and H2O. Since the H2O is hot, it is vapor. But if you hold a cold surface above the burning candle, the gaseous water will cool and condense as liquid water.

#6

And a small amount of the liberated carbon doesn’t oxidize, which is why that cold surface will also accumulate a small amount of soot. So, I suppose, some of the process of burning solid paraffin produces a solid condensate (soot).

#7

Welcome to the Straight Dope Message Boards, grempa, we’re glad you found us. For future ref, when you start a thread, it’s helpful to other readers to provide a link to the column in question. Saves search time, avoids (some) confusion, and helps keep us on the same page. No biggie, you’ll know for next time. And, as I say, welcome!