No, keys would not float on lava. Most keys are made of metal, which is much denser than molten lava. Lava has a density of about 2.7–3.3 g/cm³, while metals like brass (commonly used for keys) have a density of around 8.5 g/cm³. This means a key would sink in lava, assuming it doesn’t melt first, which could happen quickly depending on the temperature of the lava (which ranges from about 700°C to 1,250°C).