After a compound X was added to water, the pH using indicator paper was less than 7. The resulting solution was added to some calcium carbonate and it was noted that effervescence took place. What could compound X be?

Compound X would be an acid whose pH is below 7 and is able to react with calcium carbonate, a slightly basic salt.

Explanation:

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In this case, according to the given information, it turns out possible for us to firstly say that the calcium carbonate is a slightly basic salt, which is likely to react with acids and produce effervescence due to the release of carbon dioxide gas. Moreover, since the compound X exhibited a pH below 7 when dissolved in water, it is 100% sure that this compound is an acid, due to these two reasonings.

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no, the red giant is the hottest with 2.00.000.000 degrees celsius. while the white dwarf only 25,000 degrees celsius.

the answer is na (sodium)