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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nacl or sodium chloride is an ionic compound. this is because an ionic compound pertains to a compound that consists of positively and negatively charged ions. in this case, na (sodium) has a tendency to lose one electron when it reacts with with other atoms, so the resulting sodium ion contains 11 protons and 10 electrons, thus making it positively charged (na+), when chlorine reacts, it usually gains one electron to form an ion called chloride, thus making it negatively charged ( since the ions are oppositely charged, they attract.