What is Ionisation of Water (Kw)?

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Grade Level:

Class 12

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Definition

What is it?

Ionisation of water (Kw) is when pure water molecules break apart into hydrogen ions (H+) and hydroxide ions (OH-). This process shows that water, even though it seems neutral, can slightly conduct electricity because of these ions.

Simple Example

Quick Example

Imagine a glass of pure water. It looks still, but inside, a tiny fraction of water molecules are constantly playing a game of 'pass the proton'. One water molecule gives away a proton to another, forming H3O+ (which we simplify as H+) and OH-. It's like a very small, constant exchange happening all the time.

Worked Example

Step-by-Step

Let's find the concentration of H+ and OH- ions in pure water at 25 degrees Celsius.

Step 1: We know the ion product of water, Kw, is 1.0 x 10^-14 at 25 degrees Celsius.

Step 2: In pure water, the concentration of H+ ions is equal to the concentration of OH- ions. Let's call this concentration 'x'.

Step 3: The formula for Kw is [H+] * [OH-]. So, Kw = x * x = x^2.

Step 4: Substitute the value of Kw: 1.0 x 10^-14 = x^2.

Step 5: To find 'x', take the square root of both sides: x = sqrt(1.0 x 10^-14).

Step 6: Calculate the square root: x = 1.0 x 10^-7 M.

Answer: So, in pure water at 25 degrees Celsius, [H+] = 1.0 x 10^-7 M and [OH-] = 1.0 x 10^-7 M.

Why It Matters

Understanding water ionisation is crucial for fields like medicine, where maintaining the right pH in our body is vital for health. In environmental science, it helps measure water quality in rivers and lakes, impacting everything from aquatic life to climate studies. Engineers use this concept to design water treatment plants and manage industrial processes, ensuring safe and clean water for everyone.

Common Mistakes

MISTAKE: Thinking pure water has no H+ or OH- ions at all. | CORRECTION: Pure water always has a small, equal amount of H+ and OH- ions due to self-ionisation, making it neutral.

MISTAKE: Confusing Kw with Ka or Kb. | CORRECTION: Kw is specifically the ion product of water, while Ka and Kb are dissociation constants for general acids and bases.

MISTAKE: Believing Kw changes significantly with adding an acid or base. | CORRECTION: Kw remains constant at a given temperature, even if an acid or base is added. What changes are the individual concentrations of H+ and OH- ions, but their product stays Kw.

Practice Questions

Try It Yourself

QUESTION: If the concentration of H+ ions in a solution is 1.0 x 10^-3 M at 25 degrees Celsius, what is the concentration of OH- ions? | ANSWER: 1.0 x 10^-11 M

QUESTION: At a certain temperature, if Kw for water is 2.5 x 10^-14, what is the [H+] in pure water at that temperature? | ANSWER: 1.58 x 10^-7 M

QUESTION: A chemist measures the [OH-] in a sample of water to be 5.0 x 10^-9 M at 25 degrees Celsius. Is this water acidic, basic, or neutral? Explain. | ANSWER: Acidic. Since [H+] = Kw / [OH-] = (1.0 x 10^-14) / (5.0 x 10^-9) = 2.0 x 10^-6 M. Because [H+] (2.0 x 10^-6 M) is greater than [OH-] (5.0 x 10^-9 M), the water is acidic.

MCQ

Quick Quiz

What is the approximate value of the ion product of water (Kw) at 25 degrees Celsius?

1.0 x 10^-7

1.0 x 10^-14

1.0 x 10^0

1.0 x 10^1

The Correct Answer Is:

Kw = [H+][OH-], and in pure water at 25 degrees Celsius, both [H+] and [OH-] are 1.0 x 10^-7 M. So, Kw = (1.0 x 10^-7) * (1.0 x 10^-7) = 1.0 x 10^-14. Other options are incorrect values for Kw.

Real World Connection

In the Real World

When you buy bottled mineral water in India, sometimes the label mentions its pH value. This pH value is directly related to the ionisation of water and the concentrations of H+ and OH- ions. Water purification plants, like those supplying water to your home, constantly monitor Kw and pH to ensure the water is safe and potable. Even in making homemade curd or dosa batter, the pH (influenced by water ionisation) plays a role in the fermentation process!

Key Vocabulary

Key Terms

IONISATION: The process where a molecule gains or loses electrons to form ions | HYDROGEN ION (H+): A proton, often represented as H3O+ in water | HYDROXIDE ION (OH-): An ion formed when water loses a proton | ION PRODUCT OF WATER (Kw): The product of the concentrations of H+ and OH- ions in water, constant at a given temperature | NEUTRAL WATER: Water where [H+] equals [OH-]

What's Next

What to Learn Next

Great job understanding water ionisation! Next, you should explore 'pH and pOH Scale'. This will show you how we use Kw to measure how acidic or basic a solution is, which is super important for many chemical and biological processes.