Effect of pH on solubility

Summary

This video explores how pH affects the solubility of sparingly soluble salts, focusing on the impact of acid-base reactions. It explains that salts like carbonates and hydroxides react with pH changes, using calcium hydroxide as an example. Adding hydrochloric acid to a saturated solution increases solubility by reducing hydroxide concentration, prompting more salt to dissolve. The video also discusses an exam question on iron hydroxide solubility at low pH, showing how increased hydronium ions react with hydroxide ions, shifting equilibrium and enhancing solubility.

Takeaways

• 🔬 The video discusses the impact of pH on the solubility of sparingly soluble salts, a topic often considered by the New Zealand National Certificate of Educational Achievement (NCEA).
• 🧪 The solubility of a salt is affected by pH if it undergoes an acid-base reaction, which is typical for carbonates or hydroxides, except for those of sodium, potassium, or ammonium.
• 📚 Calcium hydroxide is used as an example to illustrate how adding hydrochloric acid can increase its solubility by forming water and reducing hydroxide ion concentration.
• ⚖️ The equilibrium between calcium hydroxide and its ions is disturbed by the addition of hydrogen ions, leading to more calcium hydroxide dissolving to restore balance.
• 🌡️ Changes in pH affect the autoionization of water, which influences the solubility of sparingly soluble salts like calcium hydroxide.
• 📝 The script emphasizes the importance of including both the dissolution of the salt and the reaction with hydrogen ions in the explanation.
• 📚 An NCEA exam question is analyzed, focusing on the solubility of iron hydroxide in water at a pH less than 4, where the presence of excess hydronium ions increases solubility.
• 🧐 The video provides a methodological approach to explaining the effect of pH on solubility, using equations and chemical concepts.
• 🔍 The video concludes with a teaser for the next topic, which will cover the effect of complex ions on solubility.
• 🎓 The script is educational, aiming to help viewers understand and answer exam questions related to solubility and pH.

Q & A

What is the main focus of the video?

The video focuses on the effect of pH on the solubility of sparingly soluble salts, particularly when the salt is a carbonate or hydroxide.

Why is the pH factor considered important in solubility discussions?

pH is important because it can affect the solubility of certain salts by undergoing acid-base reactions, which in turn can either add or remove hydroxide ions in solution.

Which type of salts are typically affected by pH changes in their solubility?

Salts that are carbonates or hydroxides are typically affected by pH changes in their solubility, unless they are sodium, potassium, or ammonium salts.

What happens when hydrochloric acid is added to a saturated solution of calcium hydroxide?

When hydrochloric acid is added, the hydrogen ions react with hydroxide ions, forming water and reducing the hydroxide concentration, which causes more calcium hydroxide to dissolve, thus increasing its solubility.

How does the autoionization of water relate to the solubility of sparingly soluble salts?

The autoionization of water, which is the basis of the pH scale, affects solubility by changing the concentration of hydroxide ions in solution, which can either increase or decrease the solubility of certain salts.

What is the effect of lowering the pH on the solubility of iron hydroxide?

Lowering the pH to less than 4 results in an excess of hydronium ions, which react with hydroxide ions from the dissolving iron hydroxide. This reaction shifts the equilibrium to favor the forward reaction, increasing the solubility of iron hydroxide.

What is the significance of including equations in the explanation of pH effects on solubility?

Including equations in the explanation provides a clear scientific basis for the observed effects of pH on solubility, showing the chemical reactions that occur and how they lead to changes in solubility.

Why is it important to consider both the forward and reverse reactions when discussing pH and solubility?

Considering both the forward and reverse reactions is important because it shows how the equilibrium shifts in response to changes in pH, which directly affects the solubility of the salt.

What is the role of hydronium ions in the solubility of sparingly soluble salts at low pH?

At low pH, hydronium ions react with hydroxide ions, removing them from the equilibrium and causing the equilibrium to shift to the right, which increases the solubility of the salt.

How does the video script guide viewers in understanding the relationship between pH and solubility?

The video script guides viewers by providing clear examples, such as calcium hydroxide and iron hydroxide, and explaining the chemical reactions involved, which help to illustrate the relationship between pH and solubility.

What is the purpose of the video series that this script is a part of?

The purpose of the video series is to explore various factors that affect the solubility of sparingly soluble salts, with a focus on providing explanations supported by chemical equations and real-world examples.