Acids, Bases, and Salts

TL;DR

Acids are proton donors (H$^+$), bases are proton acceptors (OH$^-$), and they react in neutralization to form salt and water. pH measures acidity/basicity, with 7 being neutral. The strength of an acid or base depends on its degree of ionisation in water.

1. The Mental Model

Think of acids and bases as chemical opposites that love to react with each other. When they meet, they cancel each other out, like positive and negative charges, creating something entirely new: a salt. This reaction is fundamental to how many chemicals behave.

2. The Core Material

Acids, bases, and salts are three fundamental types of chemical compounds. Understanding their properties and how they interact is crucial in chemistry.

What are Acids?

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Acids are substances that produce hydrogen ions (H$^+$) when dissolved in water. These H$^+$ ions are what give acids their characteristic properties.
* Properties of Acids:
* Sour taste (e.g., lemon juice) – don't taste in the lab!
* Corrosive
* Turn blue litmus paper red
* React with metals to produce hydrogen gas (e.g., Mg + 2HCl → MgCl$_2$ + H$_2$)
* React with carbonates to produce carbon dioxide gas (e.g., CaCO$_3$ + 2HCl → CaCl$_2$ + H$_2$O + CO$_2$)
* pH value less than 7

What are Bases?

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Bases are substances that produce hydroxide ions (OH$^-$) when dissolved in water. Alkalis are soluble bases.
* Properties of Bases (especially alkalis):
* Bitter taste (e.g., soap) – don't taste in the lab!
* Slippery, soapy feel
* Corrosive
* Turn red litmus paper blue
* pH value greater than 7

The pH Scale

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The pH scale is a measure of how acidic or basic a substance is. It ranges from 0 to 14.
* pH < 7: Acidic
* pH = 7: Neutral (e.g., pure water)
* pH > 7: Basic (alkaline)

Strength of Acids and Bases

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The strength of an acid or base refers to its degree of ionisation in water, not its concentration.
* Strong Acid: Completely ionises in water, producing a high concentration of H$^+$ ions (e.g., HCl, H$_2$SO$_4$).
* Weak Acid: Partially ionises in water, producing a low concentration of H$^+$ ions (e.g., CH$_3$COOH - ethanoic acid).
* Strong Base: Completely ionises in water, producing a high concentration of OH$^-$ ions (e.g., NaOH, KOH).
* Weak Base: Partially ionises in water, producing a low concentration of OH$^-$ ions (e.g., NH$_3$ - ammonia).

What are Salts?

Salts are ionic compounds formed when the hydrogen ions of an acid are replaced by a metal ion or ammonium ion. They are typically formed from the reaction of an acid and a base.
* Types of Salt Formation Reactions (Neutralisation):
* Acid + Base → Salt + Water (e.g., HCl + NaOH → NaCl + H$_2$O)
* Acid + Metal → Salt + Hydrogen gas (e.g., 2HCl + Zn → ZnCl$_2$ + H$_2$)
* Acid + Metal Carbonate → Salt + Water + Carbon dioxide (e.g., H$_2$SO$_4$ + CuCO$_3$ → CuSO$_4$ + H$_2$O + CO$_2$)
* Acid + Ammonia → Salt (e.g., HCl + NH$_3$ → NH$_4$Cl)

Here's a simple flow of how acids, bases, and salts are related through reactions:

graph TD
    A["Acid"] --> B["Neutralisation Reaction"];
    C["Base"] --> B;
    B --> D["Salt"];
    B --> E["Water"];
    F["Metal"] --> G["Reaction with Acid"];
    G --> D;
    G --> H["Hydrogen Gas"];
    I["Metal Carbonate"] --> J["Reaction with Acid"];
    J --> D;
    J --> E;
    J --> K["Carbon Dioxide"];
    L["Ammonia"] --> M["Reaction with Acid"];
    M --> D;

3. Worked Example

Let's look at a neutralisation reaction between a strong acid and a strong base: hydrochloric acid (HCl) and sodium hydroxide (NaOH).

Question: Write the chemical equation for the reaction between hydrochloric acid and sodium hydroxide. Identify the acid, base, and the salt formed.

Solution:

1. Identify the acid and base:

   • Hydrochloric acid (HCl) is an acid.
   • Sodium hydroxide (NaOH) is a base.

2. Recall the general neutralisation reaction: Acid + Base → Salt + Water.

3. Determine the ions involved:

   • HCl ionises to H$^+$ and Cl$^-$
   • NaOH ionises to Na$^+$ and OH$^-$

4. Form the salt and water:

   • The H$^+$ from the acid combines with the OH$^-$ from the base to form water (H$_2$O).
   • The remaining ions, Na$^+$ and Cl$^-$, combine to form the salt, sodium chloride (NaCl).

5. Write the balanced chemical equation:
   HCl (aq) + NaOH (aq) → NaCl (aq) + H$_2$O (l)

In this reaction:
* Acid: Hydrochloric acid (HCl)
* Base: Sodium hydroxide (NaOH)
* Salt: Sodium chloride (NaCl)

4. Key Takeaways

• Acids release H$^+$ ions in water, while bases release OH$^-$ ions (or accept H$^+$ ions).
• The pH scale indicates acidity (pH < 7), neutrality (pH = 7), or alkalinity (pH > 7).
• Neutralisation is the reaction between an acid and a base, typically forming a salt and water.
• The strength of an acid or base depends on its degree of ionisation, not its concentration.
• Salts are ionic compounds formed when a metal or ammonium ion replaces the hydrogen of an acid.

Common Mistakes to Avoid:
- Confusing strength with concentration; a dilute strong acid is different from a concentrated weak acid.
- Forgetting that not all bases are soluble in water; only soluble bases are called alkalis.
- Assuming all neutralisation reactions produce only salt and water; some also produce hydrogen gas or carbon dioxide.
- Misinterpreting litmus paper results (blue for base, red for acid).

5. Now Try It

Take a common antacid tablet (e.g., containing calcium carbonate, CaCO$_3$) and react it with a small amount of vinegar (ethanoic acid, CH$_3$COOH) in a clear glass. Observe what happens. Write down the chemical equation for this reaction. What signs indicate a chemical reaction has occurred, and how does this relate to the properties of acids and bases you've learned? What does success look like? You should be able to correctly identify the reactants, products, and explain the observable changes.