- Acids, bases, and salts are used in our daily life.
- Sour and bitter tastes of food are due to the presence of acids and bases. Acids are sour in taste, and bases are bitter.
- Natural and synthetic acid–base indicators are used to test whether a substance is an acid or a base.
Natural Indicators
- Litmus
- Red cabbage leaves
- Turmeric
- Coloured petals of some flowers (Hydrangea, Petunia, Geranium)
Litmus Solution
- Litmus solution is a purple dye extracted from lichen (a plant under division Thallophyta). When neutral, its colour is purple.
- Acids change the colour of blue litmus to red, while bases change red litmus to blue.
- A stain of curry on a white cloth turns reddish-brown when soap (basic) is rubbed on it because the curry contains the natural indicator turmeric. If washed with water, it turns yellow again.
Synthetic Indicators
- Methyl orange
- Phenolphthalein
UNDERSTANDING THE CHEMICAL PROPERTIES OF ACIDS & BASES
Acids and Bases in the Laboratory
Put a drop of each solution given below on a watch-glass one by one and test with a drop of different indicators.
- Hydrochloric acid (HCl)
- Calcium hydroxide [Ca(OH)₂]
- Sulphuric acid (H₂SO₄)
- Potassium hydroxide (KOH)
- Nitric acid (HNO₃)
- Magnesium hydroxide [Mg(OH)₂]
- Acetic acid (CH₃COOH)
- Ammonium hydroxide (NH₄OH)
- Sodium hydroxide (NaOH)
Results:
| Sample solution | Red litmus solution | Blue litmus solution | Phenolphthalein solution | Methyl orange solution |
|---|---|---|---|---|
| HCl | No change | Red | Colourless | Pinkish red |
| H₂SO₄ | No change | Red | Colourless | Pinkish red |
| HNO₃ | No change | Red | Colourless | Pinkish red |
| CH₃COOH | No change | Red | Colourless | Pinkish red |
| NaOH | Blue | No change | Red to pink | Remains yellow |
| KOH | Blue | No change | Red to pink | Remains yellow |
| Mg(OH)₂ | Blue | No change | Red to pink | Remains yellow |
| NH₄OH | Blue | No change | Red to pink | Remains yellow |
| Ca(OH)₂ | Blue | No change | Red to pink | Remains yellow |
Olfactory Indicators
These are some substances whose odour changes in acidic or basic media. E.g. onion, clove oil, and vanilla.
Test with Onion:
- Take some finely chopped onions in a plastic bag along with some clean cloth strips. Tie up the bag tightly and leave it overnight in the fridge.
- Take two of these cloth strips and check their odour.
- Put a few drops of dilute HCl solution on one strip and a few drops of dilute NaOH solution on the other.
- Rinse both cloth strips with water and again check their odour. The odour of onion cloth vanishes in NaOH. The odour remains unchanged in HCl.
Test with Vanilla Essence:
- Take some dilute HCl solution in one test tube and dilute NaOH solution in another.
- Add a few drops of dilute vanilla essence to both test tubes and shake well. The odour of vanilla essence vanishes in NaOH. The odour remains unchanged in HCl.
Test with Clove Oil:
- Repeat the same test using clove oil.
- The odour of clove oil vanishes in NaOH. The odour remains unchanged in HCl.
How do Acids & Bases React with Metals?
Reaction of Acids with Metal:
- Set the apparatus as shown in the figure.
- Take 5 mL dilute sulphuric acid in a test tube and add some zinc granules.
- Hydrogen gas bubbles form on the surface of zinc granules. Pass this gas through soap solution. Soap bubbles filled with hydrogen are formed.
- Take a burning candle near a gas-filled bubble. Hydrogen burns with a pop sound.
- Repeat this using HCl, HNO₃, and CH₃COOH.
- Bubbles come out vigorously with strong acids.
- Zinc also reacts with weak acids like acetic acid, but gas is formed slowly.
- When an acid reacts with a metal, the metal displaces hydrogen atoms from the acid as hydrogen gas and forms a compound called salt.
- However, such reactions are not possible with all metals.
Acid + Metal → Salt + H₂ gas
2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g)
(Sodium zincate)
How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?
- Take 0.5 g of sodium carbonate (Na₂CO₃) in test tube A and 0.5 g of sodium hydrogencarbonate (NaHCO₃) in test tube B. Add 2 mL dilute HCl to both the test tubes.
- CO₂ gas is produced in both test tubes.
- Test tube A:
- Test tube B:
- Pass CO₂ through lime water (calcium hydroxide).
- On passing excess CO₂:
- Limestone, chalk, and marble are different forms of calcium carbonate. All metal carbonates and hydrogen carbonates react with acids to give corresponding salt, CO₂, and water.
Na₂CO₃ + 2HCl → 2NaCl + CO₂ + H₂O
NaHCO₃ + HCl → NaCl + CO₂ + H₂O
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
(Lime water) (White precipitate)
CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)
(Soluble in water)
Metal carbonate/Metal hydrogen carbonate + Acid → Salt + CO₂ + H₂O
How do Acids and Bases React with Each Other?
- Take 2 mL dilute NaOH solution in a test tube and add two drops of phenolphthalein solution.
- The solution becomes pink colour.
- To this, add dilute HCl solution drop by drop. The pink colour changes and disappears.
- Now add a few drops of NaOH to the above mixture. The pink colour of phenolphthalein reappears.
- The effect of a base is nullified by an acid and vice-versa.
- The reaction between an acid and a base to give a salt and water is called neutralisation reaction.
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Base + Acid → Salt + H₂O
Reaction of Metallic Oxides with Acids
- Take some copper oxide (CuO) in a beaker and add dilute hydrochloric acid slowly while stirring.
- The solution becomes blue-green colour and the copper oxide dissolves. The colour is due to the formation of copper(II) chloride (CuCl₂).
- General reaction between a metal oxide and an acid is:
- Metallic oxides react with acids to give salts and water, like the reaction of a base with an acid. So metallic oxides are basic oxides.
CuO + 2HCl → CuCl₂ + H₂O
Metal oxide + Acid → Salt + H₂O
Reaction of a Non-Metallic Oxide with Base
- Calcium hydroxide (base) reacts with carbon dioxide to produce a salt and water.
- This is similar to the reaction between a base and acid. It means non-metallic oxides are acidic in nature.