2 (a) Iron reacts with oxygen to form iron oxide - Edexcel - GCSE Chemistry - Question 2 - 2011 - Paper 1

The water level in the test tube rose because:

• Iron reacts with oxygen from the air to form iron oxide.
• During the reaction, oxygen is removed from the air inside the test tube.
• As the volume of gas decreases, the pressure inside the test tube lowers, causing the water to rise and fill the space left by the consumed oxygen.

To calculate the volume of gas remaining at the end of the reaction:

1. Initial volume of air = 10 cm³.

2. Percentage of oxygen in air = 21%, which means 79% is not oxygen.

3. Therefore, the volume of air remaining is calculated as:

   $\text{Volume of air remaining} = 10 \times \frac{79}{100} = 7.9 \text{ cm}^3$

At the end of the experiment, most of the gas remaining in the test tube would be:

• A) argon
• B) carbon dioxide
• C) nitrogen
• D) oxygen (X)

Since oxygen is used up in the reaction, nitrogen is the predominant gas remaining.

The use of fossil fuels affects the composition of the atmosphere in several ways:

1. Burning fossil fuels releases carbon dioxide (CO₂) into the atmosphere, contributing to the greenhouse effect and climate change.
2. It also produces other pollutants, such as nitrogen oxides and sulfur dioxide, which can lead to acid rain and poor air quality.
3. The increase in CO₂ concentration enhances the natural greenhouse effect, leading to global warming.