No country currently backs its currency with gold, but many have in the past, including the U.S.; for half a century beginning in 1879, Americans could trade in $20.67 for an ounce of gold. The country effectively abandoned the gold standard in 1933, and completely severed the link between the dollar and gold in 1971.
Like air, water expands as it gets warmer and as a result becomes less dense. Water is most dense at temperatures near freezing. When water freezes, however, it expands, becoming less dense. If equal volumes of cold water (down to 4deg.C, 39.2deg.F) and hot water are compared, cold water weighs more than hot water.