Dissolved in nitric acid and hot concentrated sulfuric acid, it generates mercuric nitrate and mercuric sulfate, respectively, and mercurous salts appear in excess of mercury. It can dissolve many metals and form alloys, which are called amalgam. The valences are +1 and +2. Similar to silver, mercury can also react with hydrogen sulfide in the air. Mercury has a constant coefficient of volume expansion, is less metal active than zinc and cadmium, and cannot displace hydrogen from acid solutions. Generally, the valence of mercury compounds is +1 or +2. Mercury compounds with a valence of +4 have only mercury tetrafluoride, and mercury compounds with a valence of +3 do not exist.