Class 10 Science Notes Chapter 2 — Acids, Bases and Salts#
Indicators#
| Indicator | In acid | In base |
|---|---|---|
| Blue litmus | Red | No change |
| Red litmus | No change | Blue |
| Methyl orange | Red | Yellow |
| Phenolphthalein | Colourless | Pink |
| Turmeric | No change | Reddish-brown |
- Litmus is a natural indicator, extracted from lichen. Other natural indicators: turmeric, red cabbage, coloured petals (hydrangea, petunia, geranium).
- Olfactory indicators change smell. Onion and vanilla lose their smell in a base. Clove oil also loses its smell in a base.
- A curry stain (turmeric) on cloth turns reddish-brown with soap (a base), and turns yellow again when rinsed with plenty of water.
Reactions of acids and bases#
| Reaction | General equation |
|---|---|
| Acid + metal | Salt + hydrogen gas |
| Base + metal (Zn, Al) | Salt + hydrogen gas |
| Acid + metal carbonate / hydrogencarbonate | Salt + CO₂ + water |
| Acid + base (neutralisation) | Salt + water |
| Acid + metal oxide | Salt + water |
| Base + non-metal oxide | Salt + water |
$$2\text{NaOH} + \text{Zn} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2$$
$$\text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2$$
$$\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}$$
$$\text{CuO} + 2\text{HCl} \rightarrow \text{CuCl}_2 + \text{H}_2\text{O}$$
$$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$$
- Test for H₂: it burns with a pop sound when a burning candle is brought near.
- Test for CO₂: it turns lime water milky (CaCO₃ forms). Excess CO₂ makes it clear again (soluble Ca(HCO₃)₂ forms).
- Metal oxides are basic, so they react with acids. Non-metal oxides are acidic, so they react with bases.
- CuO is black. With HCl, the solution turns blue-green (copper(II) chloride).
Acids and bases in water#
- Acids give H⁺ ions in water. H⁺ cannot exist alone; it joins water to form hydronium ions, H₃O⁺.
- Bases give OH⁻ ions in water. Bases that dissolve in water are called alkalis (NaOH, KOH, Ca(OH)₂).
- Acids conduct electricity only in water. Dry HCl gas does not change dry litmus; HCl in water does.
- Diluting an acid: always add acid to water slowly, never water to acid. The process is highly exothermic, and the mixture can splash and burn.
The pH scale#
- pH measures how acidic or basic a solution is, from 0 to 14. “p” is for potenz (German for power).
| pH | Nature | H⁺ concentration |
|---|---|---|
| Less than 7 | Acidic (lower pH = stronger acid) | High |
| 7 | Neutral | — |
| More than 7 | Basic (higher pH = stronger base) | Low |
- Universal indicator is a mixture of indicators that shows a different colour at each pH.
- Strong acids give many H⁺ ions (HCl, H₂SO₄, HNO₃). Weak acids give fewer (acetic acid, citric acid).
pH in everyday life#
- Our body works at pH 7.0–7.8. Acid rain (pH below 5.6) lowers the pH of rivers and harms aquatic life.
- Soil: plants need a particular pH range to grow well.
- Digestion: the stomach makes HCl. Too much causes acidity; an antacid (a mild base like milk of magnesia, Mg(OH)₂) neutralises it.
- Tooth decay starts when the mouth’s pH falls below 5.5. Toothpastes are basic and neutralise the extra acid.
- Stings: a bee sting or nettle sting injects methanoic acid. Rubbing a mild base like baking soda helps. Dock leaves are a traditional remedy for nettle stings.
Salts#
- A salt is formed when an acid and a base neutralise each other. Salts of the same positive or negative radical form a family (sodium salts, chloride salts).
- Strong acid + strong base → neutral salt (pH 7). Strong acid + weak base → acidic salt (pH below 7). Weak acid + strong base → basic salt (pH above 7).
- Common salt (NaCl) is made from HCl and NaOH. It is found in sea water and as rock salt.
Chemicals from common salt#
Sodium hydroxide (NaOH)#
It is made by the chlor-alkali process: electricity is passed through brine (a solution of sodium chloride in water).
$$2\text{NaCl} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{Cl}_2 + \text{H}_2$$Chlorine gas forms at the anode, hydrogen gas at the cathode, and sodium hydroxide solution near the cathode. All three products are useful.
Uses: making soaps, detergents, paper and artificial fibres.
Bleaching powder (CaOCl₂)#
It is made by the action of chlorine on dry slaked lime.
$$\text{Ca(OH)}_2 + \text{Cl}_2 \rightarrow \text{CaOCl}_2 + \text{H}_2\text{O}$$Uses: bleaching cotton and linen in the textile industry and wood pulp in paper factories, bleaching washed clothes in laundry, as an oxidising agent in chemical industries, and making drinking water free of germs.
Baking soda (NaHCO₃)#
Its chemical name is sodium hydrogencarbonate. It is made from sodium chloride.
$$\begin{aligned}&\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 + \text{NH}_3 \\ &\rightarrow \text{NH}_4\text{Cl} + \text{NaHCO}_3\end{aligned}$$It is a mild, non-corrosive base. On heating, it gives off carbon dioxide, which makes cakes rise.
$$2\text{NaHCO}_3 \xrightarrow{\text{heat}} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$$Baking powder is baking soda mixed with a mild edible acid such as tartaric acid. The acid removes the bitter taste of the sodium carbonate that would otherwise form.
Uses: baking powder, antacids, and soda-acid fire extinguishers.
Washing soda (Na₂CO₃·10H₂O)#
Sodium carbonate is first obtained by heating baking soda. It is then recrystallised to give washing soda.
$$\text{Na}_2\text{CO}_3 + 10\text{H}_2\text{O} \rightarrow \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$$Uses: in the glass, soap and paper industries, making borax, cleaning at home, and removing permanent hardness of water.
Plaster of Paris (CaSO₄·½H₂O)#
It is made by heating gypsum (CaSO₄·2H₂O) at 373 K. When mixed with water, it sets into a hard mass of gypsum again.
$$\begin{aligned}&\text{CaSO}_4 \cdot \tfrac{1}{2}\text{H}_2\text{O} + 1\tfrac{1}{2}\text{H}_2\text{O} \\ &\rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}\end{aligned}$$Uses: supporting fractured bones in the right position, making toys and decorative items, and making surfaces smooth.
Water of crystallisation#
- The fixed number of water molecules in one formula unit of a salt: CuSO₄·5H₂O, Na₂CO₃·10H₂O, CaSO₄·2H₂O.
- Heating blue copper sulphate crystals removes this water, and they turn white. Adding water makes them blue again.
- The crystals look dry even though they contain water.
Common mistakes#
- Adding water to concentrated acid. Always add acid to water.
- Saying all bases are alkalis. Only water-soluble bases are alkalis.
- Thinking a higher pH means more acidic. Lower pH means more acidic.
- Writing Plaster of Paris as CaSO₄·2H₂O. That is gypsum; PoP has half a water molecule.
- Confusing baking soda (NaHCO₃) with washing soda (Na₂CO₃·10H₂O).