Acids, Bases and Salts

Collect HCl, H₂SO₄, HNO₃, CH₃COOH, NaOH, Ca(OH)₂, KOH, Mg(OH)₂ and NH₄OH from the laboratory. Put a drop of each on a watch-glass and test it with a drop of red litmus, blue litmus, phenolphthalein and methyl orange. Record your observations in Table 2.1.

(Board questions ask exactly these indicator colours — learn the table.)

Keep finely chopped onion with strips of cloth in a bag in the fridge overnight — the strips pick up the onion smell. Put dilute HCl on one strip and dilute NaOH on another: in the base (NaOH) the onion smell is lost, in the acid it stays. The smell of vanilla essence and clove oil is also destroyed in a base.

Substances whose odour changes in an acidic or basic medium are called olfactory indicators — very useful for visually impaired students.

Take zinc granules in a test tube and add dilute sulphuric acid. Fix the tube in a stand and pass the gas from the delivery tube through soap solution — gas-filled bubbles form. Bring a burning candle near the bubbles: the gas burns with a “pop” sound. It is hydrogen.

Repeat with dilute HCl, HNO₃ and CH₃COOH — record your observations. Every acid gives a salt and hydrogen gas with the metal.

Take zinc granules and NaOH solution in a test tube and warm it. Hydrogen is given off here too, and the salt sodium zincate is formed.

(Na₂ZnO₂ = sodium zincate — its negative ion is made of the metal and oxygen.) Caution: such reactions do not happen with all metals.

Take sodium carbonate (Na₂CO₃) in test tube A and sodium hydrogencarbonate (NaHCO₃) in B and add dilute HCl to both — a gas comes out with brisk effervescence. Pass it through lime water Ca(OH)₂: it turns milky. The gas is CO₂.

Why milky? Insoluble white calcium carbonate forms:

With excess CO₂ the milkiness disappears — soluble calcium hydrogencarbonate forms:

Take about 2 mL dilute NaOH in a test tube and add two drops of phenolphthalein — the solution turns pink. Now add dilute HCl drop by drop — at some point the colour disappears (the acid has cancelled the effect of the base). Add a few drops of NaOH again — the colour is pink again.

An acid and a base cancel each other's effect and form salt and water — this is a neutralisation reaction.

Take a little copper(II) oxide (black) in a beaker and add dilute HCl slowly while stirring. The solution turns bluish-green and the oxide dissolves — copper(II) chloride forms.

This is just like base + acid, so metallic oxides are basic.

Fix two nails on a rubber cork in a beaker and connect them to a 6 V battery, a bulb and a switch. Pour dilute HCl, dilute H₂SO₄, a glucose solution and alcohol into the beaker in turn.

Observation: in the acid solutions the bulb glows; in glucose and alcohol it does not.

Conclusion: acids give ions in water — the cation H⁺ — so current flows. Glucose and alcohol contain hydrogen but do not give H⁺ in water; so they are not acids (Exercise 6).

Take about 1 g NaCl in a test tube, add conc. H₂SO₄ and heat — HCl gas forms and is dried by a guard tube containing anhydrous CaCl₂. Bring dry blue litmus near this dry gas — no change. Moist blue litmus — red!

Why? H⁺ ions from HCl form only in the presence of water. H⁺ cannot exist alone — it joins a water molecule to make the hydronium ion H₃O⁺:

That is why acids do not show acidic behaviour in the absence of water (Exercise 8).

Take 10 mL water in a beaker, add a few drops of conc. H₂SO₄ and swirl slowly. Touch the base of the beaker — warm! Repeat with NaOH pellets — warm again. Mixing a concentrated acid or base with water is highly exothermic.

Test a few everyday solutions with universal indicator or pH paper and write the nature of each. The colours are only a rough guide.

Take about 2 g of soil and 5 mL of water in a test tube, shake, and filter. Read the pH of the filtrate with universal indicator paper. Most crops grow well in soil that is about neutral to slightly acidic (around 6–7). Very acidic soil is treated with lime (CaO or Ca(OH)₂) to raise the pH.

Potassium sulphate K₂SO₄, sodium sulphate Na₂SO₄, calcium sulphate CaSO₄, magnesium sulphate MgSO₄, copper sulphate CuSO₄, sodium chloride NaCl, sodium nitrate NaNO₃, sodium carbonate Na₂CO₃ and ammonium chloride NH₄Cl. Keep the charges balanced: SO₄²⁻ needs two Na⁺, but only one Ca²⁺.

Dissolve the salt in distilled water and test with litmus and pH paper. The rule is simple:

• Strong acid + strong base → neutral salt (NaCl, KNO₃). pH ≈ 7.
• Strong base + weak acid → basic salt (Na₂CO₃, NaHCO₃, CH₃COONa). pH > 7.
• Strong acid + weak base → acidic salt (NH₄Cl, CuSO₄, AlCl₃). pH < 7.

Heat a few crystals of blue copper sulphate in a dry boiling tube. The colour changes from blue to white and drops of water collect at the cool mouth of the tube. Those drops came from water inside the crystal. Put 2–3 drops of water on the cold white powder — the blue colour returns.

Water fixed in a definite proportion in the crystal lattice is called water of crystallisation. CuSO₄·5H₂O has five molecules, so it is copper sulphate pentahydrate.