D Since lime water turns milky when carbon dioxide is passed through it, Jenna can infer that her exhaled air contains carbon dioxide.

Chemical reactions are not just confined to laboratories. They are a part of our everyday lives! From the food we digest to the way we breathe, many processes around us involve chemical reactions. When we cook, our ingredients react under heat to form new flavours and aromas. Cleaning agents we use in our homes undergo reactions to break down dirt and stains.
Understanding these reactions can help us make informed choices. For example, knowing that baking a cake involves a reaction between baking soda and acids can help us adjust our recipes for better results.
Recognizing these scenarios helps us appreciate the science behind the daily tasks we often take for granted.

Our breathing process involves inhaling oxygen and exhaling carbon dioxide. But what exactly is in our exhaled air?
When we breathe in, our bodies absorb oxygen from the air through our lungs. This oxygen travels through our bloodstream to cells all over our body. These cells use oxygen for numerous processes, and in return, they produce carbon dioxide (CO₂) as a waste product.
We then exhale this CO₂ along with other gases. The composition of exhaled air typically is about 78% nitrogen, 16% oxygen, 4% carbon dioxide, and small amounts of other gases.

• Nitrogen: 78%
• Oxygen: 16%
• Carbon dioxide: 4%
• Other gases: Trace amounts

This change in composition from inhaled to exhaled air is a crucial part of our respiratory system, helping to manage the balance of gases in our body.

The lime water test is a simple but effective way to detect the presence of carbon dioxide (CO₂). Lime water, a solution of calcium hydroxide (Ca(OH)₂), reacts with CO₂ to form calcium carbonate (CaCO₃).
This reaction is represented by the equation:
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
Calcium carbonate (CaCO₃) is insoluble in water, which means it forms a milky suspension. This is why lime water turns milky when carbon dioxide is present.
Conducting the lime water test:

• Take a clear solution of lime water.
• Blow air gently through the solution using a straw.
• Observe the solution. If it turns milky, CO₂ is present.

The test not only verifies the presence of CO₂ but also demonstrates a straightforward chemical reaction. Jenna can deduce that her exhaled air contains CO₂ as it turns the lime water milky.