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Chapter 17: Problem 116

You purchase a bottle of water. On checking its pH, you find that it is not neutral as you might have expected. Instead, it is slightly acidic. Why?

Short Answer

Expert verified
The water is slightly acidic due to dissolved carbon dioxide forming carbonic acid.

Step by step solution

01

Understanding pH Scale

The pH scale measures how acidic or basic a solution is, ranging from 0 to 14. A pH of 7 is neutral, below 7 is acidic, and above 7 is basic. Water ideally should have a pH of about 7, which is neutral.
02

Characteristics of Neutral Water

Neutral water, with a pH of 7, means that the concentration of hydrogen ions (H鈦�) is equal to the concentration of hydroxide ions (OH鈦�). This balance is characteristic of pure water.
03

Reasons for Acidity in Bottled Water

There are several reasons why bottled water might be slightly acidic: carbon dioxide (CO鈧�) can dissolve in water forming carbonic acid (H鈧侰O鈧�), minerals and dissolved substances can lower pH, and contamination or bottled water treatment processes can influence acidity.
04

Carbon Dioxide Influence

When exposed to air, water can absorb carbon dioxide, which reacts with water to form carbonic acid, thus reducing pH to a slightly acidic level.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Acidic Water
When we talk about water being acidic, it means that its pH level is below 7. Naturally, pure water is neutral with a pH of 7. However, several factors can cause water to become acidic. One main reason is the presence of impurities or dissolved substances that lower the pH. Additionally, the water treatment process itself can sometimes influence the acidity of bottled water.
  • Bottled water often experiences slight acidity due to absorbing carbon dioxide from the air, which forms carbonic acid.
  • Natural mineral content and substances dissolved in the water can also contribute to its acidity.
Carbonic Acid Formation
Carbonic acid is a weak acid formed when carbon dioxide (CO鈧�) dissolves in water. This chemical reaction is quite common and is an example of how gases in the atmosphere can influence the properties of water.
When CO鈧� comes into contact with water, it reacts to form carbonic acid (H鈧侰O鈧�). This is a reversible reaction, meaning carbonic acid can also decompose back into CO鈧� and water.
  • The equation for carbonic acid formation is:
    CO鈧� + H鈧侽 鈫� H鈧侰O鈧�.

Carbonic acid lowers the pH of water, making it slightly acidic, which explains why open bottles of water can taste different over time as they absorb more CO鈧�.
Dissolved Carbon Dioxide
Carbon dioxide (CO鈧�) is a naturally occurring gas that easily dissolves in water. This process is significant as it naturally occurs when water is exposed to the open air or during carbonation in beverages.
  • This is why many carbonated drinks are acidic鈥攖hey have higher concentrations of dissolved CO鈧�.
  • The solubility of carbon dioxide in water means that even plain water can become acidic when CO鈧� is absorbed.

This dissolved CO鈧� reacts with water to form carbonic acid, which affects the pH scale by lowering it and thus increasing acidity.
Water pH Scale
The pH scale is crucial for understanding the acidity or basicity of water. It ranges from 0 to 14 and acts as a measure of the hydrogen ion concentration in a solution.
  • On this scale, a pH of 7 indicates neutral water, where hydrogen and hydroxide ions are in balance.
  • A pH less than 7 denotes acidity, while a pH greater than 7 indicates a basic or alkaline nature.

Even a small change on this scale can signify a significant shift in acidity. Hence, bottled water companies pay close attention to pH levels, as they affect taste, safety, and overall product quality. Understanding the pH scale helps consumers make informed decisions about their drinking water and its potential effects.

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Most popular questions from this chapter

The base ethylamine \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\right)\) has a \(K_{\mathrm{b}}\) of \(4.3 \times 10^{-4}\) A closely related base, ethanolamine (HOCH,CH \(_{2} \mathrm{NH}_{2}\) ), has a \(K_{\mathrm{b}}\) of \(3.2 \times 10^{-5}\) (a) Which of the two bases is stronger? (b) Calculate the \(\mathrm{pH}\) of a \(0.10 \mathrm{M}\) solution of the stronger base.

\(\begin{aligned} &\text { Consider the following ions: } \mathrm{NH}_{4}^{+}, \mathrm{CO}_{3}^{2-}, \mathrm{Br}^{-}, \mathrm{S}^{2-}, \text { and }\\\ &\mathrm{ClO}_{4}^{-} \end{aligned}\) (a) Which of these ions might lead to an acidic solution and which might lead to a basic solution? (b) Which of these anions will have no effect on the pH of an aqueous solution? (c) Which ion is the strongest base? (d) Write a chemical equation for the reaction of each basic anion with water.

Which of the following common food additives would give a basic solution when dissolved in water? (a) \(\mathrm{NaNO}_{3}\) (used as a meat preservative) (b) \(\mathrm{NaC}_{6} \mathrm{H}_{5} \mathrm{CO}_{2}\) (sodium benzoate; used as a soft-drink preservative) (c) \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\) (used as an emulsifier in the manufacture of pasteurized cheese)

Calculate the pH of the solution that results from mixing \(25.0 \mathrm{mL}\) of \(0.14 \mathrm{M}\) formic acid and \(50.0 \mathrm{mL}\) of \(0.070 \mathrm{M}\) sodium hydroxide.

Dissolving \(\mathrm{K}_{2} \mathrm{CO}_{3}\) in water gives a basic solution. Write a balanced equation showing how the carbonate ion is responsible for this effect.
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