The correct option for the question is **B: the air in the gas jar had been used up by burning**. Let's break down the reasoning behind this answer step-by-step. ### Explanation of the Correct Answer (Option B) 1. **Understanding the Setup**: - In this experiment, a gas jar is inverted over burning yellow phosphorus, which is floating on water in a beaker. When phosphorus burns, it reacts with oxygen in the air to form phosphorus pentoxide (P₂O₅), a white smoke. 2. **Combustion Reaction**: - The chemical reaction for the combustion of phosphorus can be represented as: \[ 4P + 5O_2 \rightarrow 2P_2O_5 \] - Here, phosphorus (P) reacts with oxygen (O₂) from the air to produce phosphorus pentoxide (P₂O₅). 3. **Air Composition**: - Air is composed of approximately 21% oxygen and 78% nitrogen, with small amounts of other gases. When phosphorus burns, it consumes oxygen from the air in the gas jar. 4. **Volume Change**: - As the phosphorus burns, it uses up the oxygen in the gas jar. Since the gas jar is sealed (inverted), the total volume of gas inside the jar decreases because some of the gas (oxygen) is converted into a solid (phosphorus pentoxide) and is no longer in gaseous form. 5. **Water Level Rise**: - The decrease in the volume of gas inside the jar creates a partial vacuum. The atmospheric pressure outside the jar is now greater than the pressure inside the jar. This pressure difference causes the water from the beaker to rise into the gas jar to equalize the pressure. ### Why Other Options Are Incorrect - **Option A: Pressure inside the gas jar is greater than pressure outside it**: - This option is incorrect because, after the combustion of phosphorus, the pressure inside the gas jar actually decreases due to the consumption of oxygen. Therefore, the pressure inside is not greater than outside; it is lower, which is why the water rises. - **Option C: Oxygen in the gas jar had been used up by burning**: - While it is true that oxygen is consumed during the burning of phosphorus, this option is misleading. The question asks why the water level rises, and the correct explanation involves the overall air (gas) in the jar being used up, not just the oxygen. The combustion leads to a reduction in the total gas volume, which is the primary reason for the water level rise. - **Option D: Nitrogen in the gas jar had been used up by burning**: - This option is incorrect because nitrogen does not react with phosphorus during the combustion process. The nitrogen remains in the gas jar and does not contribute to the decrease in gas volume. Therefore, it does not explain the rise in water level. ### Summary of Key Points - The burning of phosphorus consumes oxygen from the air in the gas jar, leading to a decrease in gas volume. - This decrease creates a partial vacuum, causing the water level to rise in the gas jar. - The correct answer is that the air (specifically the oxygen) in the gas jar is used up, leading to the observed phenomenon. - Understanding the relationship between gas volume, pressure, and combustion reactions is crucial in explaining this experiment. By grasping these concepts, students can better understand the principles of gas behavior and chemical reactions in a laboratory setting.