5. Acids, Bases and Salts

Ammonium (NH₄⁺) is a positively charged basic radical (cation). Chloride (Cl⁻), nitrate (NO₃⁻) and hydride (H⁻) are negatively charged acidic radicals (anions).

Hydrogen chloride gives H⁺ ions in water, so it is an acid. Sodium hydroxide, calcium oxide and ammonia give OH⁻ ions in water, so they are bases.

Carbonic acid (H₂CO₃) gives two H⁺ ions per molecule, so it is dibasic. Acetic acid (CH₃COOH), hydrochloric acid (HCl) and nitric acid (HNO₃) give one H⁺ ion each, so they are monobasic.

Ammonium chloride is formed from a strong acid (HCl) and a weak base (NH₄OH), so it is an acidic salt (pH less than 7). The other three are formed from strong acids and strong bases, so they are neutral salts (pH 7).

Sodium carbonate is formed from a weak acid (carbonic acid) and a strong base (NaOH), so it is a basic salt (pH more than 7). The other three are neutral salts of strong acids and a strong base.

Zinc oxide reacts with both acids and bases, so it is an amphoteric oxide. Calcium oxide, magnesium oxide and sodium oxide react only with acids, so they are basic oxides.

Common salt (NaCl) crystals have no water of crystallisation. Blue vitriol (CuSO₄·5H₂O), ferrous sulphate (FeSO₄·7H₂O) and sodium carbonate (Na₂CO₃·10H₂O) all contain water of crystallisation.

Acetic acid is a weak acid that dissociates only slightly in water, so it is a weak electrolyte. Sodium chloride and sodium acetate (salts) and potassium hydroxide (a strong base) dissociate almost completely, so they are strong electrolytes.

Change: the blue colour of the solution becomes lighter. Reason: the quantity of copper sulphate stays the same but the volume of the solution doubles, so its concentration becomes half. The solution becomes more dilute, and the colour, which depends on the concentration of Cu²⁺ ions, fades.

Change: the colourless solution turns pink. Reason: phenolphthalein is colourless in acidic and neutral solutions but pink in basic solutions. Sodium hydroxide is a strong base, and its solution contains a large number of OH⁻ ions.

Change: the copper filings slowly dissolve, the solution turns blue, and a colourless gas is given off which turns brown on coming in contact with air. Reason: copper reacts with nitric acid to form copper nitrate, Cu(NO₃)₂, whose solution is blue because of Cu²⁺ ions.

Change: in dilute HCl the litmus paper turns red. After concentrated NaOH is added and stirred, it turns blue. Reason: HCl is an acid, and acids turn blue litmus red. The concentrated NaOH contains far more OH⁻ ions than there are H⁺ ions in the dilute acid. It first neutralizes all the acid (H⁺ + OH⁻ → H₂O), and the excess base then makes the solution basic, which turns red litmus blue.

Change: magnesium oxide dissolves in dilute HCl, but there is no change in dilute NaOH. Reason: magnesium oxide is a basic oxide. It reacts with an acid to form a salt and water, but it does not react with a base.

Change: zinc oxide dissolves in both. Reason: zinc oxide is an amphoteric oxide — it reacts with an acid as well as with a base, forming a salt and water in each case.

Change: brisk effervescence is seen and the limestone slowly dissolves. The gas turns limewater milky. Reason: limestone is calcium carbonate. Acids react with carbonates to give a salt, water and carbon dioxide gas; the bubbles are of CO₂.

Change: on heating, the blue crystals turn into a white powder and drops of water collect on the cooler upper part of the test tube. When water is added after cooling, the white powder turns blue again. Reason: blue vitriol contains five molecules of water of crystallisation. Heating drives this water out, the crystal structure breaks down, and the blue colour is lost. Adding water restores the hydrated form and its blue colour. These are physical changes.

Change: bubbles of gas form at both electrodes. The volume of gas collected at the cathode is double that at the anode. Reason: dilute H₂SO₄ dissociates into ions, so the water conducts electricity and electrolysis of water takes place. Hydrogen gas is formed at the cathode and oxygen gas at the anode, in the volume ratio 2 : 1.

• A sodium atom (2, 8, 1) has one electron in its outermost shell. It gives away this electron and becomes the sodium ion Na⁺ (2, 8), which has a complete octet.
• A chlorine atom (2, 8, 7) has seven electrons in its outermost shell. It takes up the electron given by sodium and becomes the chloride ion Cl⁻ (2, 8, 8), which also has a complete octet.
• The oppositely charged Na⁺ and Cl⁻ ions attract each other. This force of attraction is one ionic bond, and the stable ionic compound sodium chloride, NaCl, is formed.

• A magnesium atom (2, 8, 2) has two electrons in its outermost shell. It gives away both and becomes the magnesium ion Mg²⁺ (2, 8), with a complete octet.
• Each chlorine atom (2, 8, 7) can take up only one electron. So two chlorine atoms are needed; each takes one electron and becomes a chloride ion Cl⁻ (2, 8, 8).
• The Mg²⁺ ion attracts the two Cl⁻ ions, forming two ionic bonds. The ionic compound magnesium chloride, MgCl₂, is formed.

Strong acids, strong bases and salts dissociate almost completely in water, so the proportion of dissociation is large. Weak acids (acetic acid) and weak bases (ammonia) dissociate only partly; most of their molecules remain undissociated, so the proportion is small.

Concentration (g/L) = mass of solute in g ÷ volume of solution in L; molarity (mol/L) = concentration in g/L ÷ molecular mass.

The basicity of an acid is the number of H⁺ ions obtained by the dissociation of one molecule of the acid. On this basis acids are of three types:

The reaction in which an acid reacts with a base to form a salt and water is called neutralization. In it, the H⁺ ions of the acid combine with the OH⁻ ions of the base to form water: H⁺ + OH⁻ → H₂O. For example, HCl + NaOH → NaCl + H₂O.

• Acidity in the stomach: when excess acid in the stomach causes discomfort, an antacid such as milk of magnesia (magnesium hydroxide), a base, is taken. It neutralizes the excess acid.
• Acidic soil: when the soil in a field becomes too acidic, farmers add lime (calcium oxide) or slaked lime, which are basic. They neutralize the acid in the soil.
• Other examples: toothpaste, which is basic, neutralizes the acid formed in the mouth that causes tooth decay; baking soda applied to an ant or bee sting neutralizes the acid injected.

Pure water is a bad conductor of electricity because it has very few ions. When a little salt, strong acid or strong base is dissolved in it and an electric current is passed, water decomposes into its constituent elements, hydrogen and oxygen. This decomposition of water by electric current is called electrolysis of water.

• At the cathode (negative electrode): water molecules take up electrons coming from the cathode. Hydrogen gas is released, and OH⁻ ions are formed, so the solution near the cathode becomes basic (turns red litmus blue).
• At the anode (positive electrode): water molecules give up electrons to the anode. Oxygen gas is released, and H⁺ ions are formed, so the solution near the anode becomes acidic (turns blue litmus red).
• For every 4 electrons, 2 molecules of hydrogen but only 1 molecule of oxygen are formed. So the volume of hydrogen collected at the cathode is double the volume of oxygen at the anode.

1. Definition: an acid gives H⁺ as the only cation on dissolving in water, whereas a base gives OH⁻ as the only anion.
2. Taste and touch: acids are sour; bases are bitter and feel slippery or soapy.
3. Litmus: acids turn blue litmus red; bases turn red litmus blue.
4. Phenolphthalein: it stays colourless in acids but turns pink in bases.
5. pH: an acidic solution has pH less than 7; a basic solution has pH more than 7.
6. Examples: acids — HCl, H₂SO₄, CH₃COOH; bases — NaOH, KOH, Ca(OH)₂.

1. Charge: a cation is a positively charged ion, whereas an anion is a negatively charged ion.
2. Formation: a cation is formed when an atom gives away electrons; an anion is formed when an atom takes up electrons.
3. Type of radical: cations are basic radicals; anions are acidic radicals.
4. Usually formed from: cations mostly from metals (exception NH₄⁺); anions mostly from non-metals (exception MnO₄⁻).
5. In electrolysis: cations move towards the cathode; anions move towards the anode.
6. Examples: cations — Na⁺, Ca²⁺, Al³⁺; anions — Cl⁻, O²⁻, SO₄²⁻.

1. Name: the negative electrode is called the cathode; the positive electrode is called the anode.
2. Connection: the cathode is connected to the negative terminal of the battery; the anode is connected to the positive terminal.
3. Ions attracted: the cathode attracts cations; the anode attracts anions.
4. Electrons: at the cathode, electrons coming from the battery are taken up by the ions or molecules in the solution; at the anode, electrons are removed from the solution (or from the anode metal) and sent to the battery.
5. Examples: in the electrolysis of copper sulphate, copper is deposited on the cathode while the copper anode dissolves. In the electrolysis of water, hydrogen is released at the cathode and oxygen at the anode.

• Common salt and potassium bromide are salts of a strong acid and a strong base, so they are neutral.
• Sodium acetate and sodium carbonate are salts of a weak acid and a strong base (basic salts). Calcium hydroxide and ammonia are bases.
• Ammonium chloride is a salt of a strong acid and a weak base (acidic salt). Carbon dioxide and sulphur dioxide are non-metal oxides that form acids in water. HCl and vinegar (acetic acid) are acids.

Electroplating is the use of electrolysis to deposit a thin layer of one metal on an article made of another metal. The article is made the cathode, a plate of the coating metal is the anode, and a solution of a salt of the coating metal is the electrolyte. It protects articles from rusting and corrosion and makes them look attractive.

Ordinary rainwater usually shows a pH of about 5.6 to 6.5, so it is slightly acidic. This is because carbon dioxide in the air dissolves in rainwater to form weak carbonic acid. Where the air contains oxides of sulphur and nitrogen from factories and vehicles, the rain becomes more strongly acidic (pH below 5.6); this is called acid rain.

Effects of acid rain: it makes soil acidic and less fertile; damages the leaves of plants and forests; makes lakes and rivers acidic, harming fish and other aquatic life; corrodes buildings, statues and monuments made of marble and limestone, and metal structures; and can irritate the skin and eyes of people and animals.