Which Statement Best Describes The PH Of Pure Water?A. It Is Neutral Because The Concentration Of Hydronium Ions Equals That Of Hydroxide Ions.B. It Is Neutral Because The Pure Liquid Contains Neither Hydronium Ions Nor Hydroxide Ions.C. It Is Acidic

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Understanding the pH of Pure Water: A Key Concept in Chemistry

Introduction

The pH of a solution is a measure of its acidity or basicity, and it is a fundamental concept in chemistry. Pure water is a unique substance that has a pH of 7 at room temperature, which is considered neutral. However, understanding why pure water has a pH of 7 requires a deeper dive into the chemistry of water and its ions.

The pH Scale

The pH scale is a logarithmic scale that ranges from 0 to 14, with 7 being the neutral point. A pH less than 7 is considered acidic, while a pH greater than 7 is considered basic. The pH scale is defined as the negative logarithm of the concentration of hydronium ions (H3O+) in a solution.

The Chemistry of Pure Water

Pure water is composed of two hydrogen atoms and one oxygen atom (H2O). At room temperature, pure water is a neutral substance, meaning it does not have a significant concentration of hydronium ions or hydroxide ions (OH-). However, when pure water is exposed to air, it can absorb carbon dioxide (CO2) from the atmosphere, which reacts with water to form carbonic acid (H2CO3). This reaction is as follows:

CO2 + H2O → H2CO3

The carbonic acid then dissociates into hydronium ions and bicarbonate ions (HCO3-):

H2CO3 → H3O+ + HCO3-

This reaction shows that pure water can contain a small concentration of hydronium ions, even though it is considered neutral.

The Concentration of Hydronium Ions in Pure Water

The concentration of hydronium ions in pure water is extremely low, typically on the order of 10^-7 moles per liter (M). This is because the dissociation of water into hydronium and hydroxide ions is a very slow process, and it is also a reversible reaction. The equilibrium constant for this reaction is known as the water dissociation constant (Kw), which is approximately 10^-14 at room temperature.

The Concentration of Hydroxide Ions in Pure Water

The concentration of hydroxide ions in pure water is also extremely low, typically on the order of 10^-7 M. This is because the dissociation of water into hydronium and hydroxide ions is a very slow process, and it is also a reversible reaction.

The pH of Pure Water

Given the extremely low concentrations of hydronium and hydroxide ions in pure water, it may seem counterintuitive that the pH of pure water is 7. However, the pH of a solution is defined as the negative logarithm of the concentration of hydronium ions, not the concentration of hydroxide ions. Since the concentration of hydronium ions in pure water is 10^-7 M, the pH of pure water is indeed 7.

Conclusion

In conclusion, the pH of pure water is 7 because the concentration of hydronium ions is 10^-7 M, which is the same as the concentration of hydroxide ions. This is a result of the water dissociation reaction, which is a reversible reaction that occurs very slowly. The pH of pure water is a fundamental concept in chemistry, and it is essential to understand the chemistry of water and its ions to appreciate this concept.

Frequently Asked Questions

  • Q: What is the pH of pure water? A: The pH of pure water is 7.
  • Q: Why is the pH of pure water 7? A: The pH of pure water is 7 because the concentration of hydronium ions is 10^-7 M, which is the same as the concentration of hydroxide ions.
  • Q: What is the water dissociation constant (Kw)? A: The water dissociation constant (Kw) is approximately 10^-14 at room temperature.

References

  • "Chemistry: The Central Science" by Theodore L. Brown, H. Eugene LeMay, and Bruce E. Bursten
  • "General Chemistry: Principles and Modern Applications" by Linus Pauling
  • "The Chemistry of Water" by the United States Environmental Protection Agency

Further Reading

  • "The pH Scale" by the American Chemical Society
  • "The Chemistry of Acids and Bases" by the Royal Society of Chemistry
  • "The Water Cycle" by the National Oceanic and Atmospheric Administration (NOAA)
    Frequently Asked Questions: Understanding the pH of Pure Water

Introduction

The pH of pure water is a fundamental concept in chemistry, and it is essential to understand the chemistry of water and its ions to appreciate this concept. In this article, we will answer some of the most frequently asked questions about the pH of pure water.

Q&A

Q: What is the pH of pure water?

A: The pH of pure water is 7.

Q: Why is the pH of pure water 7?

A: The pH of pure water is 7 because the concentration of hydronium ions is 10^-7 M, which is the same as the concentration of hydroxide ions.

Q: What is the water dissociation constant (Kw)?

A: The water dissociation constant (Kw) is approximately 10^-14 at room temperature.

Q: Why is the pH of pure water not 0?

A: The pH of pure water is not 0 because the concentration of hydronium ions is extremely low, typically on the order of 10^-7 M. If the concentration of hydronium ions were 0, the pH would be 0.

Q: Why is the pH of pure water not 14?

A: The pH of pure water is not 14 because the concentration of hydroxide ions is also extremely low, typically on the order of 10^-7 M. If the concentration of hydroxide ions were 0, the pH would be 14.

Q: Can the pH of pure water change?

A: Yes, the pH of pure water can change if it is exposed to air, which contains carbon dioxide (CO2). The CO2 can react with water to form carbonic acid, which can dissociate into hydronium and bicarbonate ions.

Q: How does the pH of pure water relate to the acidity or basicity of a solution?

A: The pH of pure water is a reference point for measuring the acidity or basicity of a solution. A solution with a pH less than 7 is considered acidic, while a solution with a pH greater than 7 is considered basic.

Q: What is the significance of the pH of pure water in everyday life?

A: The pH of pure water is significant in everyday life because it affects the taste, odor, and color of water. For example, water with a pH that is too low or too high can be unpleasant to drink.

Conclusion

In conclusion, the pH of pure water is a fundamental concept in chemistry that is essential to understand the chemistry of water and its ions. By answering some of the most frequently asked questions about the pH of pure water, we hope to have provided a better understanding of this concept.

Frequently Asked Questions: Additional Resources

  • "The pH Scale" by the American Chemical Society
  • "The Chemistry of Acids and Bases" by the Royal Society of Chemistry
  • "The Water Cycle" by the National Oceanic and Atmospheric Administration (NOAA)
  • "Chemistry: The Central Science" by Theodore L. Brown, H. Eugene LeMay, and Bruce E. Bursten
  • "General Chemistry: Principles and Modern Applications" by Linus Pauling
  • "The Chemistry of Water" by the United States Environmental Protection Agency

Further Reading

  • "Acid-Base Chemistry" by the University of California, Berkeley
  • "The pH of Water" by the National Institute of Standards and Technology (NIST)
  • "Water Chemistry" by the American Water Works Association (AWWA)