Acids, Bases & Salts

Acids are sour, bases are bitter and soapy, neutral substances are like pure water.

An acid releases positive hydrogen ions (H⁺); a base releases negative hydroxide ions (OH⁻).

No. An alkali is a base that dissolves in water — every alkali is a base, but not every base is an alkali.

A salt (plus water) — table salt NaCl is one member of a huge family.

An acid releases positive hydrogen ions (H+) when dissolved in water; the word comes from Latin acidus, meaning sour.

An ion is an atom or group of atoms carrying an electric charge.

H₃O⁺. A bare H⁺ never floats freely — it grabs water, and H₃O⁺ is what actually makes a solution acidic.

An acid is a proton (H⁺) donor; a base is a proton acceptor.

An acid accepts an electron pair; a base donates an electron pair — the widest definition.

Acid/base behaviour with litmus: acids turn blue litmus red, bases turn red litmus blue.

The modern water-based definition — an acid gives H⁺ ions in water, a base gives OH⁻ ions in water. This is the most tested definition.

Acid

Gives H+

Base

Gives OH-

Acid

Proton donor

Base

Proton acceptor

Acid

Electron-pair acceptor

Base

Electron-pair donor

Svante Arrhenius

• Svante Arrhenius (1880s) gave the modern water-based definition: acid gives H+ ions in water, base gives OH- ions in water (the most tested definition).

Taste sour, turn blue litmus red, react with active metals to give hydrogen gas, and are corrosive.

Taste bitter, feel soapy/slippery, turn red litmus blue; strong ones are corrosive too.

Only in water do molecules split into free ions — acids conduct via moving H⁺, bases via moving OH⁻.

With no water it cannot form ions or H₃O⁺. Add a drop of water and the colour changes at once.

Properties appear only when dissolved in water, because only then do molecules split into free ions.

Litmus indicator colour changes

How completely a substance ionises in water — nothing else. Strong acids: HCl, H₂SO₄, HNO₃. Weak: acetic, citric. Strong bases: NaOH, KOH. Weak: NH₄OH, Mg(OH)₂.

No. Concentration is how much acid is in the water; strength is how completely it ionises. A dilute strong acid is still strong, and a concentrated weak acid is still weak.

Only about 1% — yet it is clearly sour. That is what makes it a weak acid.

A strong acid ionises almost completely, releasing a flood of H+ ions (HCl, H2SO4, HNO3).

Strong bases (NaOH, KOH) ionise fully; weak bases (NH4OH) ionise only partly.

Strong examples

HCl, H2SO4, HNO3

Weak examples

Acetic, citric

Strong examples

NaOH, KOH

Weak examples

NH4OH, Mg(OH)2

Acetic acid (CH₃COOH). Its pure water-free form is called glacial acetic acid.

Citric acid, lactic acid, and tartaric acid (tartaric is also in banana).

Oxalic acid, formic acid (methanoic acid), and malic acid.

HCl, H₂SO₄, HNO₃ — they come from minerals, are strong and corrosive, and are used in labs and factories. Organic acids, by contrast, come from living things and are usually weak and safe to eat.

Acid present

Acetic acid

Acid present

Citric acid

Acid present

Lactic acid

Acid present

Tartaric acid

Acid present

Oxalic acid

Acid present

Formic acid

Acid present

Malic acid

Caustic soda, caustic potash, baking soda, washing soda.

Slaked lime (used in whitewash), milk of magnesia (a mild antacid), and ammonia solution.

Sodium hydroxide (NaOH), caustic soda, is a very strong alkali used to make soap and clean drains; dangerously corrosive.

Baking soda (NaHCO3) is a mild base used in cooking and antacids.

Washing soda (Na2CO3) is used for cleaning.

Magnesium hydroxide, milk of magnesia, is a mild antacid for an acidic stomach.

Formula

NaOH

Formula

KOH

Formula

NaHCO3

Formula

Na2CO3

Formula

Ca(OH)2

Formula

Mg(OH)2

Formula

NH4OH

A natural purple dye obtained from lichens. Memory hook: 'Acid to Red'; a base does the opposite.

Methyl orange: red in acid, yellow in base. Phenolphthalein: colourless in acid, pink in base. Turmeric: yellow in acid, red/brown in base.

Nothing — distilled water leaves red as red and blue as blue.

Onion, clove, vanilla — they change smell rather than colour with acids and bases.

An indicator changes colour to tell us whether something is acidic or basic, like a traffic light for chemistry.

In acid

Red

In base

Blue

In acid

Red

In base

Yellow

In acid

Colourless

In base

Pink

In acid

Yellow

In base

Red/brown

How acidic or basic a solution is, from 0 to 14. Below 7 is acidic, exactly 7 is neutral (pure water), above 7 is basic.

Each step of 1 pH unit means a 10-times change in H⁺ concentration. So a rise from pH 3 to pH 6 means H⁺ falls 1000 times.

About 1 — pH = −log(0.1) = 1, since a strong acid is nearly fully ionised.

Lower pH = more acidic (more H+ ions); higher pH = more basic (more OH- ions).

A universal indicator shows the full pH range as a spread of colours.

Nature

Acidic

Example

Lemon (2), vinegar

Nature

Neutral

Example

Pure water

Nature

Basic

Example

Soap, NaOH

pH scale (0-14)

Salt + hydrogen gas (Zn + dilute HCl → ZnCl₂ + H₂). Hydrogen is tested with a lit splinter that burns with a 'pop' sound.

Salt + water + CO₂ — and the CO₂ turns limewater milky, confirming the gas.

Salt + water, because metal oxides are basic in nature.

Acid + carbonate releasing CO2 (limewater test)

Acid + base → salt + water (HCl + NaOH → NaCl + H₂O); the acidic and basic properties cancel out. Antacids neutralise excess stomach acid; toothpaste neutralises mouth acids.

Oxides that react with BOTH acids and bases to give salt and water — aluminium oxide (Al₂O₃) and zinc oxide (ZnO). CuO is basic, not amphoteric.

General equation: HCl + NaOH gives NaCl + H2O.

Metal oxides are basic; non-metal oxides are acidic (e.g. CO2 forms carbonic acid in water).

Neutral salt (NaCl, pH 7) · acidic salt (NH₄Cl, pH below 7) · basic salt (Na₂CO₃, pH above 7).

The fixed water inside a salt's crystal, e.g. blue copper sulphate CuSO₄·5H₂O.

Caustic soda, baking soda, washing soda and bleaching powder all use NaCl as raw material.

A salt is formed when an acid reacts with a base; its name comes from the parent acid and base.

Acid + Base

Strong + strong

Nature

Neutral

Acid + Base

Strong + weak

Nature

Acidic

Acid + Base

Weak + strong

Nature

Basic

NaHCO₃ releases CO₂ on heating, which makes dough rise. It is also used in antacids and fire extinguishers.

CaOCl₂, made by passing chlorine over slaked lime; used to disinfect water and bleach cloth.

CaSO₄·½H₂O, made by heating gypsum (CaSO₄·2H₂O). It sets hard with water and is used to fix fractured bones.

Copper sulphate (CuSO₄·5H₂O) — used as a fungicide and in electroplating.

Washing soda (Na2CO3.10H2O) is used to remove hardness of water and for cleaning.

Gypsum is CaSO4.2H2O; heating it gives Plaster of Paris.

Blue vitriol is copper sulphate (CuSO4.5H2O), used in fungicides.

Common name / use

Baking soda; antacid

Common name / use

Washing soda; cleaning

Common name / use

Bleaching powder

Common name / use

Plaster of Paris

Common name / use

Blue vitriol

About 7.4 — living things survive only in a narrow pH range.

pH 5.5, when mouth bacteria produce acid; basic toothpaste neutralises it.

Below 5.6 — it harms plants, soil and buildings.

A bee sting is acidic (soothed by baking soda); a wasp sting is basic.

For digestion. Excess causes acidity, treated with an antacid base like baking soda or milk of magnesia.

Plants need a specific soil pH; farmers add lime (basic) to neutralise acidic soil.