3 (a) Complete the sentence by putting a cross (X) in the box next to your answer - Edexcel - GCSE Chemistry - Question 3 - 2013 - Paper 1

One major reason for the decrease in carbon dioxide levels is the increase in photosynthesis by plants. Over time, plants have absorbed more carbon dioxide from the atmosphere, utilizing it to produce oxygen and glucose. Additionally, the advancement of technology has led to more efficient energy sources, reducing carbon emissions.

Carbon dioxide and other greenhouse gases trap infrared radiation emitted from the Earth's surface, creating a greenhouse effect. This process helps maintain the Earth's temperature, preventing it from becoming too cold.

To test for oxygen, a glowing wooden splint can be introduced into a test tube or container with the gas. If the splint reignites, it indicates the presence of oxygen, as oxygen supports combustion.

The water level in the bell jar rose because the magnesium reacted with the oxygen in the air, resulting in a decrease in the volume of gas in the jar. As the volume of the gas decreased, the water rose to fill the space left by the consumed gas.

Given that the initial volume of air is 1000 cm³ and 21% of that is oxygen, the volume of oxygen initially present is:

$ext{Volume of oxygen} = 1000 imes 0.21 = 210 ext{ cm}^3$

As some oxygen is used up during the reaction with magnesium, it is essential to determine what percentage of oxygen remains. Assuming remaining gases account for 79%, the volume of gas left in the bell jar can be calculated as follows:

$ext{Volume remaining} = 1000 imes 0.79 = 790 ext{ cm}^3$

The salt formed when magnesium oxide reacts with dilute sulfuric acid is magnesium sulfate.