Acids, bases and indicators (KCSE Chemistry Form 2)

TL;DR

Acids are sour, corrosive substances that turn blue litmus red, while bases are bitter, soapy, and turn red litmus blue. Indicators are special dyes that change colour depending on whether a solution is acidic or basic. Understanding these helps you classify everyday substances and predict their reactions.

1. The Mental Model

Think of acids and bases as chemical opposites, like hot and cold. Indicators are like tiny thermometers that tell you which one you have by changing colour. This helps us identify and handle them safely.

2. The Core Material

What are Acids?

Acids are substances that produce hydrogen ions (H$^+$) when dissolved in water. They have a sour taste (think lemon juice!) and are corrosive, meaning they can eat away at materials.

Properties of Acids:
* Taste: Sour (e.g., vinegar, citrus fruits). Never taste chemicals in the lab!
* Feel: Can feel sticky or sting on the skin.
* Effect on litmus paper: Turns blue litmus paper red.
* Reaction with metals: React with reactive metals (like magnesium, zinc) to produce hydrogen gas and a salt.
* Example: Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen gas
Mg(s) + 2HCl(aq) → MgCl$_2$(aq) + H$_2$(g)
* Reaction with carbonates/bicarbonates: React to produce carbon dioxide gas, water, and a salt.
* Example: Calcium carbonate + Hydrochloric acid → Calcium chloride + Water + Carbon dioxide
CaCO$_3$(s) + 2HCl(aq) → CaCl$_2$(aq) + H$_2$O(l) + CO$_2$(g)
* Reaction with bases: Neutralise bases to form salt and water.

Common Acids:
* Strong Acids: Hydrochloric acid (HCl), Sulphuric acid (H$_2$SO$_4$), Nitric acid (HNO$_3$). These fully dissociate (break apart) in water.
* Weak Acids: Ethanoic acid (CH$_3$COOH, found in vinegar), Carbonic acid (H$_2$CO$_3$, found in fizzy drinks). These only partially dissociate in water.

What are Bases?

Bases are substances that produce hydroxide ions (OH$^-$) when dissolved in water. Bases that are soluble in water are called alkalis. They have a bitter taste and feel soapy or slippery.

Properties of Bases/Alkalis:
* Taste: Bitter (e.g., soap). Never taste chemicals in the lab!
* Feel: Soapy or slippery.
* Effect on litmus paper: Turns red litmus paper blue.
* Reaction with acids: Neutralise acids to form salt and water.
* Reaction with ammonium salts: When heated with ammonium salts, they produce ammonia gas.
* Example: Sodium hydroxide + Ammonium chloride → Sodium chloride + Water + Ammonia gas
NaOH(aq) + NH$_4$Cl(s) → NaCl(aq) + H$_2$O(l) + NH$_3$(g)

Common Bases/Alkalis:
* Strong Alkalis: Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Calcium hydroxide (Ca(OH)$_2$). These fully dissociate in water.
* Weak Alkalis: Ammonia solution (NH$_3$(aq)). This only partially dissociates in water.

What are Indicators?

Indicators are special organic dyes that change colour depending on the acidity or alkalinity of a solution. They don't tell you how strong an acid or base is, just if it's an acid or a base.

How Indicators Work:
Indicators are weak acids or bases themselves. They have different colours in their acidic and basic forms. When you add them to a solution, they react and change to the colour of their dominant form.

Common Indicators and Their Colour Changes:

Universal Indicator:
This is a mixture of several indicators. It shows a range of colours depending on the pH (a measure of acidity/alkalinity) of the solution, giving a more precise idea of how acidic or basic something is.

Identifying Acids and Bases Using Indicators

graph TD
    A[Start with a solution] --> B{Add a few drops of indicator};
    B --> C{What colour is it?};
    C -- Turns Red --> D[It's an Acid (e.g., using Litmus or Methyl Orange)];
    C -- Turns Blue --> E[It's a Base (e.g., using Litmus)];
    C -- Turns Pink --> F[It's a Base (e.g., using Phenolphthalein)];
    C -- Stays Colourless --> G[It's an Acid or Neutral (e.g., using Phenolphthalein)];
    C -- Turns Yellow --> H[It's a Base (e.g., using Methyl Orange)];
    D --> I[End];
    E --> I;
    F --> I;
    G --> I;
    H --> I;

Neutralisation

This is the reaction between an acid and a base to form a salt and water. The acid and base cancel each other out, making the resulting solution neutral (neither acidic nor basic).

• Acid + Base → Salt + Water
  • Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H$_2$O(l)
    (Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water)

3. Worked Example

You are given three unlabelled solutions: Solution A, Solution B, and Solution C. You are also provided with red litmus paper, blue litmus paper, and phenolphthalein indicator. Describe how you would identify which solution is acidic, which is basic, and which is neutral.

Step-by-step identification:

1. Test with Litmus Paper:

   • Dip a piece of blue litmus paper into Solution A. If it turns red, Solution A is acidic.
   • If it stays blue, dip a piece of red litmus paper into Solution A. If it turns blue, Solution A is basic.
   • If neither litmus paper changes colour, Solution A is neutral.
   • Repeat this process for Solution B and Solution C.

   Let's say:
   * Solution A turns blue litmus red. (Acidic)
   * Solution B turns red litmus blue. (Basic)
   * Solution C causes no change to either litmus paper. (Neutral)

2. Confirm with Phenolphthalein (Optional, but good practice):

   • Add a few drops of phenolphthalein to Solution A (which we suspect is acidic). It should remain colourless.
   • Add a few drops of phenolphthalein to Solution B (which we suspect is basic). It should turn pink.
   • Add a few drops of phenolphthalein to Solution C (which we suspect is neutral). It should remain colourless.

Conclusion:
* Solution A is acidic.
* Solution B is basic.
* Solution C is neutral.

4. Key Takeaways

• Acids taste sour, are corrosive, and turn blue litmus red.
• Bases taste bitter, feel soapy, and turn red litmus blue.
• Alkalis are bases that dissolve in water.
• Indicators are dyes that change colour to show if a solution is acidic or basic.
• Neutralisation is the reaction between an acid and a base, forming salt and water.

Common Mistakes to Avoid:
- Tasting chemicals: Never taste any chemical in the lab, even if you know its properties.
- Confusing litmus colours: Remember "A-R-B" (Acid-Red-Blue) for the colour change of blue litmus.
- Assuming all bases are alkalis: Only soluble bases are called alkalis.
- Using only one indicator: Sometimes, using a second indicator can confirm your initial observation.

5. Now Try It

You have three unknown liquids: lemon juice, soapy water, and pure water. Using only red and blue litmus paper, describe the steps you would take to correctly identify each liquid. What would you observe for each? Your answer should clearly state the observations and the conclusion for each liquid.