Acids,Bases and Salts

Take one test tube at a time and test it with red litmus.

- The test tube in which red litmus turns blue contains the basic solution.
- The test tube in which red litmus shows no change may contain either distilled water or an acidic solution, because red litmus does not change in acid or neutral solution.
- Now use the test tube identified as basic. Dip the same red litmus into the other two test tubes.
- If it turns blue there, that test tube is the base.
- If it does not change, that test tube is either acidic solution or distilled water.
- To separate these two, first dip a fresh strip of red litmus into one of them. The acidic solution will keep red litmus red, and the distilled water will also keep it red.

So, with only red litmus, you can identify the base directly, but you cannot distinguish acid from distilled water using red litmus alone. To identify all three uniquely, you need an additional indicator or prior identification of the base.

Curd and sour substances should not be kept in brass and copper vessels because they contain acids. The acids react with these metals to form harmful salts, which can contaminate the food and may be poisonous. Hence, such substances are stored in non-reactive containers.

When an acid reacts with a metal, the gas usually liberated is hydrogen gas.

Example:
$$\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\uparrow$$

To test for hydrogen, bring a burning candle or a burning splint near the gas. It burns with a 'pop' sound.

The gas that extinguishes a burning candle is carbon dioxide. Since the product formed is calcium chloride, the metal compound A must be calcium carbonate.

Balanced equation:
$$\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2\uparrow$$

HCl, HNO₃, etc. show acidic character in aqueous solution because they produce hydrogen ions in water. These ions actually exist as hydronium ions, H₃O⁺.

Alcohol and glucose contain hydrogen, but they do not produce H⁺/H₃O⁺ ions in water, so they do not show acidic character.

An aqueous solution of an acid conducts electricity because acids produce ions in water. The hydrogen ions (present as H₃O⁺) and the accompanying anions carry electric current through the solution.

Dry HCl gas does not change the colour of dry litmus paper because hydrogen ions are produced only in the presence of water. Without water, HCl does not ionise to give H⁺/H₃O⁺, so it cannot show acidic behaviour.

While diluting an acid, acid should be added to water, not water to acid, because the process is highly exothermic. Adding acid slowly to water with stirring allows the heat to be absorbed safely. If water is poured into concentrated acid, sudden heating may cause splashing and burns.

When an acid is diluted, the concentration of hydronium ions decreases because the same amount of acid is spread over a larger volume of water.

When excess base is dissolved in sodium hydroxide solution, the concentration of hydroxide ions increases because more OH⁻ ions are added to the solution.

The exact colour and pH values are approximate and can be recorded as follows:

| Solution | Nature |
|---|---|
| Saliva (before meal) | slightly acidic / near neutral |
| Saliva (after meal) | more acidic |
| Lemon juice | acidic |
| Colourless aerated drink | acidic |
| Carrot juice | slightly acidic |
| Coffee | acidic |
| Tomato juice | acidic |
| Tap water | neutral |
| 1M NaOH | basic |
| 1M HCl | strongly acidic |

On the basis of pH paper, acids show pH less than 7, neutral substances show pH 7, and bases show pH more than 7.

Solution A has pH 6 and solution B has pH 8.

- Lower pH means higher hydrogen ion concentration.
- So A has more hydrogen ion concentration than B.
- A is acidic because its pH is less than 7.
- B is basic because its pH is more than 7.

The greater the concentration of H⁺(aq) ions, the more acidic the solution becomes. Thus, increasing H⁺ ion concentration lowers the pH.

Yes, basic solutions also have H⁺(aq) ions, but their concentration is very low compared with OH⁻ ions. They are basic because the solution contains more hydroxide ions than hydrogen ions.

A farmer would treat the soil with quick lime, slaked lime or chalk when the soil is too acidic. These substances are basic and help neutralise excess acidity in the soil.

The chemical formulae are:
- Potassium sulphate — $\text{K}_2\text{SO}_4$
- Sodium sulphate — $\text{Na}_2\text{SO}_4$
- Calcium sulphate — $\text{CaSO}_4$
- Magnesium sulphate — $\text{MgSO}_4$
- Copper sulphate — $\text{CuSO}_4$
- Sodium chloride — \text{NaCl
- Sodium nitrate — $\text{NaNO}_3$
- Sodium carbonate — $\text{Na}_2\text{CO}_3$
- Ammonium chloride — $\text{NH}_4\text{Cl}$

The salts can be obtained from these acids and bases:

- Potassium sulphate — sulphuric acid and potassium hydroxide
- Sodium sulphate — sulphuric acid and sodium hydroxide
- Calcium sulphate — sulphuric acid and calcium hydroxide
- Magnesium sulphate — sulphuric acid and magnesium hydroxide
- Copper sulphate — sulphuric acid and copper hydroxide / copper oxide
- Sodium chloride — hydrochloric acid and sodium hydroxide
- Sodium nitrate — nitric acid and sodium hydroxide
- Sodium carbonate — carbonic acid and sodium hydroxide
- Ammonium chloride — hydrochloric acid and ammonium hydroxide

The salts listed can be grouped into families based on common radicals:

- Sodium salts: Na₂SO₄, NaCl, NaNO₃, Na₂CO₃
- Sulphate salts: K₂SO₄, Na₂SO₄, CaSO₄, MgSO₄, CuSO₄
- Chloride salts: NaCl, NH₄Cl

So, three families can be clearly identified from the given salts.

Using litmus and pH paper, the salts can be classified as follows:

- Neutral salts: sodium chloride, potassium nitrate
- Acidic salts: aluminium chloride, zinc sulphate, copper sulphate
- Basic salts: sodium acetate, sodium carbonate, sodium hydrogencarbonate

General idea:
- Salts formed from a strong acid and strong base are neutral.
- Salts formed from a strong acid and weak base are acidic.
- Salts formed from a strong base and weak acid are basic.

Acid/base used to form them:
- NaCl: HCl + NaOH
- KNO₃: HNO₃ + KOH
- AlCl₃: HCl + Al(OH)₃
- ZnSO₄: H₂SO₄ + Zn(OH)₂ / ZnO
- CuSO₄: H₂SO₄ + CuO / Cu(OH)₂
- CH₃COONa: CH₃COOH + NaOH
- Na₂CO₃: H₂CO₃ + NaOH
- NaHCO₃: H₂CO₃ + NaOH

The common name of $\text{Ca(ClO)}_2$ is bleaching powder.