Chemistry
Hard steel contains

0.5 to 1.5 per cent carbon
2 to 5 per cent carbon
0.1 to 0.4 per cent carbon
0.01 to 0.04 per cent carbon

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Chemistry
Aqua regia is a 1 : 3 mixture, by volume, of

Conc. nitric acid and conc. hydrochloric acid
Conc. sulphuric acid and conc. nitric acid
Conc. hydrochloric acid and conc. nitric acid
Conc. nitric acid and conc. sulphuric acid

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