Chemical Properties of Bases
🔄 Quick Recap
In the previous section, we learned about the chemical properties of acids. We discovered that:
- Acids react with metals to produce hydrogen gas and salts
- Acids react with metal carbonates to produce carbon dioxide, salt, and water
- Acids react with metal oxides to produce salt and water
Now, let's explore the chemical properties of bases!
📚 Physical Properties of Bases
Before we dive into the chemical reactions, let's understand some physical properties of bases:
- Taste: Bases taste bitter (but NEVER taste chemicals in a laboratory!)
- Touch: Bases feel slippery or soapy when dissolved in water
- Effect on indicators:
- Turn red litmus paper blue
- Turn phenolphthalein pink
- Turn methyl orange yellow
📚 Reaction of Bases with Metals
Some metals, particularly the more reactive ones, can react with bases to produce hydrogen gas.
When sodium hydroxide reacts with zinc, sodium zincate and hydrogen gas are produced:
2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) + H₂(g)
Similarly, with aluminum:
2NaOH(aq) + 2Al(s) + 2H₂O(l) → 2NaAlO₂(aq) + 3H₂(g)
However, not all metals react with bases. Only metals that can form amphoteric oxides (oxides that react with both acids and bases) like zinc, aluminum, and lead react with bases.
📚 Reaction of Bases with Non-metallic Oxides
Non-metallic oxides are acidic in nature. When bases react with non-metallic oxides, they form salt and water. This is similar to the reaction between an acid and a base.
For example, when carbon dioxide (a non-metallic oxide) is passed through lime water (calcium hydroxide solution):
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
This is why lime water turns milky when carbon dioxide is passed through it - the calcium carbonate formed is insoluble and appears as a white precipitate.
📚 Reaction of Bases with Acids
When bases react with acids, they neutralize each other to form salt and water. This is called a neutralization reaction.
For example, when sodium hydroxide reacts with hydrochloric acid:
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Or when calcium hydroxide reacts with nitric acid:
Ca(OH)₂(aq) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + 2H₂O(l)
📚 Reaction of Bases with Ammonium Salts
When bases are heated with ammonium salts, they produce ammonia gas, which has a characteristic pungent smell.
For example, when sodium hydroxide is heated with ammonium chloride:
NaOH(aq) + NH₄Cl(s) → NaCl(aq) + H₂O(l) + NH₃(g)
Testing for Ammonia Gas
You can test for ammonia gas by:
- Its characteristic pungent smell
- Turning moist red litmus paper blue
- Forming white fumes when a glass rod dipped in hydrochloric acid is brought near it
🌍 Real-Life Applications of Bases
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Soap making: Sodium hydroxide (a strong base) is used in the saponification process to make soap.
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Food preparation: Baking soda (sodium bicarbonate) is a weak base used in cooking.
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Cleaning agents: Many household cleaners contain bases because they break down grease and oils.
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Agriculture: Lime (calcium oxide or calcium hydroxide) is added to acidic soils to neutralize them.
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Medicine: Magnesium hydroxide (milk of magnesia) and aluminum hydroxide are used as antacids to neutralize excess stomach acid.
⚠️ Common Misconceptions
Misconception 1: All bases are strong and corrosive
Not all bases are strong and corrosive:
- Strong bases like sodium hydroxide and potassium hydroxide are indeed corrosive
- Weak bases like baking soda and borax are mild and relatively safe to handle
Misconception 2: All bases dissolve in water
Not all bases dissolve well in water:
- Bases that dissolve in water are called alkalis (like sodium hydroxide)
- Some bases like magnesium hydroxide are not very soluble in water
- Metal oxides like copper oxide (CuO) are bases but do not dissolve in water
🧪 Activity Time! Make Your Own Indicator
Red cabbage can be used as a natural indicator to test for bases:
Materials needed:
- Fresh red cabbage
- Hot water
- Different household substances to test
Steps:
- Chop the red cabbage into small pieces
- Add hot water and let it steep for 10 minutes
- Strain the liquid - this is your indicator!
- Test different substances:
- The indicator will turn green, blue, or even yellow with bases
- It will turn red or pink with acids
This works because red cabbage contains anthocyanins, which change color at different pH levels.
✅ Solved Example
Problem: If 20 mL of 0.1 M NaOH solution is exactly neutralized by 10 mL of an HCl solution, what is the molarity of the HCl solution?
Solution:
Step 1: Recall the balanced equation.
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Step 2: Use the relationship: Molarity₁ × Volume₁ = Molarity₂ × Volume₂ For a neutralization reaction where the acid and base react in a 1:1 ratio.
Step 3: Substitute the known values. 0.1 M × 20 mL = Molarity_HCl × 10 mL
Step 4: Solve for the molarity of HCl. Molarity_HCl = (0.1 M × 20 mL) / 10 mL = 0.2 M
Therefore, the molarity of the HCl solution is 0.2 M.
💡 Key Points to Remember
- Bases taste bitter and feel slippery when dissolved in water
- Bases turn red litmus blue
- Some metals (like zinc and aluminum) react with bases to produce hydrogen gas
- Non-metallic oxides react with bases to form salt and water
- Acids and bases neutralize each other to form salt and water
- Bases react with ammonium salts to produce ammonia gas
- Bases that dissolve in water are called alkalis
🧠 Memory Trick
To remember the reaction of bases:
- Metals like zinc give hydrogen (M for Metals)
- Acids are neutralized to salt and water (A for Acids)
- Non-metallic oxides form salts (N for Non-metallic)
- Ammonium salts release ammonia (A for Ammonium)
Together, these form "MANA" - a word you can associate with the "magic" of base reactions!
🤔 Think About It!
- Why do some metals react with bases while others don't?
- What happens when we add too much base to neutralize an acid solution?
- Why do we use lime (calcium oxide) on agricultural fields?
🔜 What Next?
Now that we've learned about the chemical properties of both acids and bases, in the next section we'll focus specifically on neutralization reactions and their important applications in our daily lives.