What is the reason for the emission of light after the gas in a discharge tube is excited?

Which statement describes how phase changes can be diagrammed as a substance is heated? the phase is on the y-axis and the temperature is on the x-axis. the temperature is on the y-axis and the phase is on the x-axis. the time is on the y-axis and the temperature is on the x-axis. the temperature is on the y-axis and the time is on the x-axis.

Match the name of the following compound:   mgso4  · h2omagnesium sulfate monohydratemagnesium (ii) sulfate monohydratemagnesium (ii) sulfate hydratemagnesium sulfate hydrate

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