The upper atmosphere of Venus, that is, the region that begins as the top of the troposphere, comprises a layer of sulfuric acid (H2SO4) haze with layers of sulfuric acid clouds floating above it. The haze begins at an altitude of roughly 30 km, and the clouds lie between about 50 and about 70 km above the surface. Sulfuric acid has such a strong affinity for water that it can remove water from a substance even when that substance does not contain any molecular water, by removing hydrogen and oxygen atoms in a 2:1 ratio. The photograph above shows what happens when you mix sucrose, C12H22O11 (table sugar) with sulfuric acid. The sulfuric acid dehydrates the sugar and becomes hydrated in the process:
C12H22O11(s) + 11H2SO4(conc) → 12C(s) + 11H2SO4·H2O(l)
This reaction is exothermic, and much of the water is driven off as steam, which pushes the carbon out of the beaker as a pillar of foam. After you pour the acid into the sugar, stir thoroughly. If you start with dry sugar, it will take a minute or two for the reaction to generate enough carbon and heat to start pushing up a foam pillar. You will first see the sugar start to become brown, then the mixture will begin to darken further, and the carbon pillar will come rising out of the beaker.
This provides a graphic illustration of how destructive the upper atmosphere of Venus would be to any kind of living organism that happened to enter it (and that you do not want to get concentrated sulfuric acid on your skin!).
The demonstration uses 70 g sugar and 70 ml sulfuric acid. A tall form 300-ml beaker is ideal, but a 400-ml griffin beaker (shown) does well.
References:
1) Chaisson, Eric and McMillan, Steve. Astronomy Today (Upper Saddle River, New Jersey: Prentice-Hall, 1999) pp. 215-217.
2) Zumdahl, Steven S. Chemical Principles (Boston: Houghton-Mifflin, 1998) pp. 874-875.
3) McQuarrie, Donald A. and Rock, Peter A. General Chemistry (New York: W.H. Freeman and Company, 1984) pp. 591-592.
4) Shakhashiri, Bassam Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vol. 1. (University of Wisconsin Press: 1983) pp. 77-78, cited in https://www.chem.indiana.edu/wp-content/uploads/2017/11/16-1-The-Dehydration-of-Sugar-by-Sulfuric-Acid.doc, available by clicking the “Download” button on the page The Dehydration of Sugar by Sulfuric Acid for the amounts of material to use in the demonstration (70 g sugar and 70 ml sufuric acid).