The correct option is **C. Oxygen in the gas jar had been used up by burning**. ### Detailed Explanation: 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 that can absorb moisture. 2. **What Happens During the Reaction**: - When yellow phosphorus burns, it consumes oxygen from the air inside the gas jar. The chemical reaction can be represented as: \[ 4P + 5O_2 \rightarrow 2P_2O_5 \] - As the phosphorus burns, it uses up the oxygen present in the gas jar. This reaction reduces the amount of gas (specifically, oxygen) in the jar. 3. **Effect on Pressure**: - According to the ideal gas law (PV = nRT), if the number of moles of gas (n) decreases while the temperature (T) and volume (V) remain constant, the pressure (P) inside the gas jar will also decrease. - As the oxygen is consumed, the pressure inside the gas jar drops below the atmospheric pressure outside the jar. 4. **Water Level Rise**: - The decrease in pressure inside the gas jar creates a pressure difference between the inside and outside of the jar. The higher atmospheric pressure outside the jar pushes the water up into the jar to equalize the pressure. This is why the water level rises in the gas jar. ### Why Other Options Are Incorrect: - **A. Pressure inside the gas jar is greater than pressure outside it**: - This option is incorrect because, as explained, the burning of phosphorus consumes oxygen, leading to a decrease in pressure inside the jar, not an increase. Therefore, the pressure inside cannot be greater than outside. - **B. The air in the gas jar had been used up by burning**: - While it is true that the air (which contains oxygen) is used up, this option is vague. It does not specify that it is the oxygen that is being consumed, which is the key factor in causing the water level to rise. The air is not entirely used up; rather, it is specifically the oxygen that is consumed. - **D. Nitrogen in the gas jar had been used up by burning**: - This option is incorrect because nitrogen is not consumed in the burning of phosphorus. Nitrogen is an inert gas in this context and does not react with phosphorus. Therefore, it does not contribute to the change in pressure or the rise of the water level. ### Summary of Key Points: - The burning of yellow phosphorus consumes oxygen in the gas jar. - This consumption leads to a decrease in pressure inside the jar. - The resulting pressure difference causes water to rise into the jar. - The other options either misinterpret the reaction or incorrectly state the role of gases involved. This thorough understanding of the reaction and the principles of gas behavior will help you grasp similar concepts in chemistry.