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Question Video: Finding the Mass of the Products in a Thermal Decomposition Reaction Science

A student placed a small quantity of zinc carbonate into a test tube. She weighed the test tube and zinc carbonate and found their mass to be 55 g. She then heated the test tube strongly for 5 minutes. After the test tube had cooled down, she reweighed it and the remaining powder inside. What is the new mass of the test tube and zinc compound? [A] More than 55 g [B] Less than 55 g [C] 55 g

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Video Transcript

A student placed a small quantity of zinc carbonate into a test tube. She weighed the test tube and zinc carbonate and found their mass to be 55 grams. She then heated the test tube strongly for five minutes. After the test tube had cooled down, she reweighed it and the remaining powder inside. What is the new mass of the test tube and zinc compound? (A) More than 55 grams, (B) less than 55 grams, or (C) 55 grams.

The question tells us that the test tube and the zinc carbonate originally weighed 55 grams. The test tube was then heated, and after it had cooled down, it was reweighed. Zinc is a metal, so zinc carbonate is a metal carbonate. We need to know what happens when a metal carbonate is heated. Many types of compounds decompose when heated. This is called thermal decomposition, which can be defined as the breaking down of a substance when heated to form two or more other substances. When a metal carbonate undergoes thermal decomposition, it forms a metal oxide and carbon dioxide. The capital Δ symbol is used to show that heat was involved.

The carbonate in the question is zinc carbonate. So, when heated, it will form zinc oxide and carbon dioxide. So we now know that the products of the reaction are zinc oxide and carbon dioxide. But to work out what the measured mass would be, we need to consider the law of conservation of mass. This states that the mass of a closed system cannot change. So, if this reaction took place in a sealed container, the mass of the starting material, zinc carbonate, would be the same as the mass of the products, zinc oxide and carbon dioxide, in which case the mass of zinc oxide and carbon dioxide produced combined with the mass of the test tube would also equal 55 grams.

But this is not a closed system, as the reaction takes place in a test tube and the question doesn’t specify that it’s sealed. So, when the zinc carbonate is heated, forming zinc oxide and carbon dioxide, the carbon dioxide is released into the atmosphere, as it’s a gas. So, when the test tube is reweighed at the end of the experiment, the carbon dioxide will not be included in the mass. Instead, only the zinc oxide and test tube are weighed. If the zinc oxide, carbon dioxide, and test tube equals 55 grams, then only the zinc oxide and test tube must be less than 55 grams. Since this is what is weighed, the answer to the question “What is the new mass of the test tube and zinc compound?” is (B) less than 55 grams.

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