Chemical Properties of Acids

🔄 Quick Recap

In the previous section, we learned that:

• Acids produce hydrogen ions (H⁺) in water solutions
• These hydrogen ions give acids their characteristic properties
• Acids can conduct electricity because of these ions

Now, let's explore how acids react with different substances!

📚 Reaction of Acids with Metals

When acids react with metals, hydrogen gas is produced along with a corresponding salt.

The General Reaction

Metal + Acid → Salt + Hydrogen gas

For example, when zinc metal reacts with dilute hydrochloric acid:

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

Or when magnesium reacts with sulphuric acid:

Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)

🧪 Activity: Testing for Hydrogen Gas

You can test for hydrogen gas by bringing a burning candle near the gas (carefully, with adult supervision!):

• Hydrogen gas burns with a characteristic "pop" sound
• The flame will show a blue-colored flash

Which Metals React with Acids?

Not all metals react with acids at the same rate:

• Very reactive metals like potassium and sodium react violently with acids
• Moderately reactive metals like zinc, iron, and magnesium react steadily
• Less reactive metals like copper, silver, and gold don't react with dilute acids

This reactivity follows the reactivity series of metals: K > Na > Ca > Mg > Al > Zn > Fe > Pb > Cu > Ag > Au

📚 Reaction of Acids with Metal Carbonates and Bicarbonates

When acids react with metal carbonates or bicarbonates, they produce a salt, carbon dioxide gas, and water.

The General Reaction

Metal carbonate + Acid → Salt + Carbon dioxide + Water
Metal bicarbonate + Acid → Salt + Carbon dioxide + Water

For example, when sodium carbonate reacts with hydrochloric acid:

Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)

Or when sodium bicarbonate (baking soda) reacts with hydrochloric acid:

NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)

Testing for Carbon Dioxide

The carbon dioxide produced can be tested using lime water (calcium hydroxide solution):

• When CO₂ is passed through lime water, it turns milky
• This happens because calcium carbonate (which is white and insoluble) is formed:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

If excess carbon dioxide is passed, the milkiness disappears due to the formation of soluble calcium bicarbonate:

CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)

📚 Reaction of Acids with Metal Oxides

Metal oxides are basic in nature. When acids react with metal oxides, they form salt and water.

The General Reaction

Metal oxide + Acid → Salt + Water

For example, when copper oxide reacts with dilute hydrochloric acid:

CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)

The blue-green color of the solution indicates the formation of copper(II) chloride.

🌍 Real-Life Applications

1. Food preservation: Vinegar (containing acetic acid) is used in pickling because many bacteria cannot survive in acidic environments.

2. Cleaning metals: Mild acids are used to remove rust (iron oxide) from iron objects.

3. Soil treatment: Farmers test their soil for acidity. If the soil is too acidic, they add lime (calcium oxide) to neutralize the acid.

4. Indigestion relief: Antacids contain bases that neutralize excess stomach acid to relieve indigestion.

5. Metal extraction: Acids are used in the extraction and purification of many metals.

⚠️ Common Misconceptions

Misconception 1: All acids react the same way with all metals

The reaction depends on both the acid and the metal:

• Some metals (like copper) don't react with dilute acids
• Concentrated nitric acid (HNO₃) behaves differently and often produces nitrogen oxides instead of hydrogen

Misconception 2: All acids are equally strong

Acids vary in strength:

• Strong acids like HCl, H₂SO₄, and HNO₃ ionize completely in water
• Weak acids like acetic acid (CH₃COOH) ionize only partially

🧪 Activity Time! Volcano Experiment

Create a model volcano using the reaction between acid and carbonate:

Materials needed:

• Baking soda (sodium bicarbonate)
• Vinegar
• Red food coloring
• Dish soap
• A model volcano (can be made from clay or paper mache)

Steps:

1. Place 2-3 tablespoons of baking soda in the crater of your model volcano
2. Add a few drops of red food coloring and a small amount of dish soap
3. Pour vinegar into the crater and watch the "eruption"!

The reaction produces carbon dioxide gas, which creates bubbles with the dish soap, causing the "lava" to flow.

✅ Solved Example

Problem: When 3g of magnesium ribbon reacts with excess hydrochloric acid, what volume of hydrogen gas would be produced at STP? (Atomic mass of Mg = 24)

Solution:

Step 1: Write the balanced equation.

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Step 2: Calculate moles of magnesium. Moles of Mg = Mass/Molar mass = 3g/24g/mol = 0.125 mol

Step 3: From the equation, 1 mol of Mg produces 1 mol of H₂. So, 0.125 mol of Mg produces 0.125 mol of H₂.

Step 4: At STP, 1 mol of any gas occupies 22.4 liters. Volume of H₂ = 0.125 mol × 22.4 L/mol = 2.8 L

Therefore, 3g of magnesium will produce 2.8 liters of hydrogen gas at STP.

💡 Key Points to Remember

• Acids react with active metals to produce hydrogen gas and a salt
• Acids react with metal carbonates and bicarbonates to produce carbon dioxide, salt, and water
• Acids react with metal oxides to produce salt and water
• Carbon dioxide turns lime water milky
• Hydrogen gas burns with a "pop" sound
• The reactivity of a metal with acids depends on its position in the reactivity series

🤔 Think About It!

1. Why don't gold and silver react with most acids?
2. Why does a copper vessel get damaged if vinegar is stored in it for a long time?
3. How can we use the reactions of acids to test for the presence of carbonates in a sample?

🔜 What Next?

Now that we've learned about the chemical properties of acids, in the next section we'll explore the chemical properties of bases and how they react with different substances.