Which model below represents a pure substance?

In 2002, the rare earth elements mine in mountain pass, california was closed because

Word equation for k(s) +h2o(l) yield koh (aq) + h2

Large helium-filled balloons are used to lift scientific equipment to high altitudes. what is the pressure inside such a balloon if it starts out at sea level with a temperature of 10.0ºc and rises to an altitude where its volume is twenty times the original volume and its temperature is – 50.0ºc ?