The correct option is **B. Magnesium**. ### Detailed Explanation: When a substance is heated in air, it can undergo a chemical reaction with oxygen, which can lead to an increase in mass. This is particularly true for metals like magnesium. 1. **Understanding the Reaction**: - When magnesium is heated in air, it reacts with oxygen to form magnesium oxide (MgO). The reaction can be represented by the following balanced chemical equation: \[ 2 \text{Mg (s)} + \text{O}_2 \text{(g)} \rightarrow 2 \text{MgO (s)} \] - In this reaction, solid magnesium (Mg) combines with gaseous oxygen (O₂) from the air to produce solid magnesium oxide (MgO). 2. **Mass Increase**: - Before the reaction, you have the mass of magnesium alone. After the reaction, you have the mass of magnesium oxide, which includes the mass of both magnesium and the oxygen that has been incorporated into the product. - Since oxygen is added to the magnesium during the reaction, the total mass of the product (magnesium oxide) is greater than the mass of the original magnesium alone. This is why we say that the mass increases when magnesium is heated in air. ### Why the Other Options Are Incorrect: **A. Helium**: - Helium is a noble gas and does not react with oxygen or any other element under normal conditions, even when heated. Therefore, its mass will not change when heated in air. It remains constant because it does not form any new compounds. **C. Copper pyrites**: - Copper pyrites (chalcopyrite, CuFeS₂) is a mineral that contains copper, iron, and sulfur. When heated in air, it can undergo oxidation, but the process is complex and typically involves the loss of sulfur as sulfur dioxide (SO₂) gas. This means that while some mass may be lost due to gas formation, the overall mass does not increase. Instead, it may decrease or remain relatively constant depending on the specific conditions. **D. Glass**: - Glass is primarily composed of silica (SiO₂) and other compounds. When heated, glass does not react with oxygen in a way that would increase its mass. Instead, it may soften and change its physical state, but its mass remains unchanged because it does not gain any additional material from the air. ### Summary of Key Points: - **Magnesium reacts with oxygen** when heated, forming magnesium oxide and increasing in mass. - **Helium does not react** with air, so its mass remains unchanged. - **Copper pyrites may lose mass** due to gas formation when heated, rather than gaining mass. - **Glass does not react** with air to gain mass when heated. ### Revision Summary: - Heating magnesium in air results in a mass increase due to the formation of magnesium oxide. - Helium remains unchanged as it does not react with oxygen. - Copper pyrites may lose mass due to gas release during heating. - Glass does not gain mass when heated in air.