Quan used a beaker to prepare a solution consisting of 4 g of a reddish powder and 35 g of alcohol. As soon as the substances mixed together, he covered the beaker tightly so that no gas could escape. After the powder dissolved, he noticed a black solid fell to the bottom of the test tube. As part of his observation, he weighed the solution with the black solid. What did he find?

Explanation:

The formation of a black solid that fell to the bottom of the test tube is the indication of a chemical reaction: the 4 g of reddish powder and the 35 g of alcohol reacted with each other and formed new substances.

The chemical reactions obey the law of conservation of mass: the mass of the reactants is equal to the mass of the products.

Since the beaker was tightly covered, none of the substances escaped, and all the products are present inside the beaker along with the reactants that were left over, in case some of them did not react to completion.

Therefore, the mass of the content of the beaker, after reaction, must be equal to the mass of the initial substances: 4 g  + 35 g = 39g.

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