sentence: Selenium is made by oxidizing selenide ores to selenium dioxide.
sentences_before: It is made as a byproduct when refining copper and certain other sulfide ores.
sentences_after: The selenium dioxide is dissolved in acidic water to make selenous acid, which is reacted with sulfur dioxide to make selenium as an element. It is the red form that is made. To make the black form, the red form is heated and melted.
concept_name: selenium
headings: Selenium Preparation
categories: Chemical elements Nonmetals