An Acid is a substance that gives H+ ions when dissolved in water.
Acids are compounds that contain Hydrogen (Hydrochloric, HCl; Sulphuric, H2SO4; Nitric, HNO3). However, not all compounds that contain Hydrogen are acids (Water, H2O; Methane, CH4). Acids are usually compounds of non metals with Hydrogen and sometimes Oxygen.
The three acids above react with water in the following ways:
HCl ---- (H2O) ----> H+ + Cl-
H2SO4 ---- (H2O) ----> 2H+ + SO42-
HNO3 ---- (H2O) ----> H+ + NO3-
Sulphuric, Hydrochloric and Nitric acids are inorganic. There are also organic acids. Acetic acid (found in vinegar) has the formula CH3CO2H. Not all the Hydrogen atoms give H+ ions in water. In acetic acid, only the Hydrogen attached to the Oxygen yield a H+ ion.
CH3CO2H ---- (H2O) ----> H+ + CH3CO2-
Many acids only show acidic properties when water is present.
Acids are corrosive and can burn flesh and dissolve metal.
Bases and Alkalis
A Base is a substance that gives OH- ions when dissolved in water.
Bases are usually metal hydroxides (MOH). Examples include Sodium Hydroxide, NaOH, Calcium Hydroxide, Ca(OH)2. The solution of a base in water is called an alkali.
The two bases above react with water in the following ways:
NaOH ---- (H2O) ----> Na+ + OH-
Ca(OH)2 ---- (H2O) ----> Ca2+ + 2OH-
Alkalis have a soapy feel and can corrode.
Acidity is measured on a scale called pH. The value of pH determines how acidic or basic a solution is. A pH of 1 is very acidic; a pH of 14 is a strong alkali. A neutral solution (or pure water) has a pH of 7.
There are chemicals that change colour at different pH values. These are called indicators. One of the most famous is Litmus. This substance turns red when the pH is less than 7 (acidic) and turns blue when the pH is greater than 7 (basic).
A Salt results when an acid reacts with a base. Both are neutralised. The H+ and OH- ions combine to form water. The non metalic ions of the acid and the metal ions of the base form the salt.
Acid + Base ----> Salt + Water
HCl + NaOH ----> NaCl + H2O
H2SO4 + Ca(OH)2 ----> CaSO4 + H2O
NaCl is Sodium Chloride (common salt); CaSO4 is Calcium Sulphate.
The salt ions normally stay in solution. The salt crystalizes out when the water is removed. Some salts are insoluble. They will precipitate out when the acid and base are added together. Barium Chloride behaves like this:
2HCl + Ba(OH)2 ----> 2H2O + BaCl2
Barium Chloride (BaCl2) appears as a white precipitate.
Salts can be formed in other ways. When metals dissolve in acids, a salt is formed along with Hydrogen:
Acid + Metal ----> Salt + Hydrogen
In the example below Magnesium dissolves in Sulphuric Acid to give Magnesium Sulphate and Hydrogen which appears in the form of bubbles:
H2SO4 + Mg ----> MgSO4 + H2
Many metal carbonates are unstable. When they dissolve in acids, a salt is formed along with water and Carbon Dioxide:
Acid + Metal Carbonate ----> Salt + Water + Carbon Dioxide
In the example below Calcium Carbonate dissolves in Hydrochloric Acid:
2HCl + CaCO3 ----> CaCl2 + H2O + CO2
Calcium Carbonate has many forms (chalk, marble and limestone) which are used for building. These substances are corroded by even weak acids as in acid rain.
Oxides are compounds containing Oxygen combined with a single element. Oxides can be Acidic, Basic or Neutral.
Acidic Oxides react with water to form an acid. Sulphur Trioxide (SO3) reacts with water to form Sulphuric Acid:
SO3 + H2O ----> H2SO4
Other acidic oxides include SO2, CO2, NO and NO2.
Basic Oxides react with water to form an alkali. Sodium Oxide is a good example:
Na2O + H2O ----> 2 NaOH
Most metal oxides are basic. Ammonia (NH3) behaves like a basic oxide when it reacts with water to form a weak alkali, Ammonium Hydroxide (NH4OH). This substance ionises as follows in water:
NH4OH ---- (H2O) ----> NH4+ + OH-
Neutral Oxides are either insoluble in water or do not form acids or alkalis when dissolved. Carbon Monoxide (CO), Dinitrogen Oxide (N2O) and water are neutral.
Acidic Oxides and Basic Oxides react together to form a Salt:
Acidic Oxide + Basic Oxide ----> Salt
In the example below, Sulphur Trioxide combines with Sodium Oxide to form the salt, Sodium Sulphate:
SO3 + Na2O ----> Na2SO4
Some metals are more reactive than others. If a metal is added to a salt solution of a less reactive metal, a displacement reaction occurs. The added metal will replace the metal in the salt. This reaction can be summarised as:
Metal A + Salt of Metal B ----> Salt of Metal A + Metal B
In the example below: Iron filings are added to a blue solution of Cupper Sulphate. Iron is more reactive than Copper. The solution turns green as the Iron forms Iron Sulphate. Copper is deposited as a red powder.
Fe + CuSO4 ----> FeSO4 + Cu
The following table shows the reactivity of selected metals (compared to Hydrogen as a standard). Generally speaking, a higher metal in the table will displace a lower metal from its compounds. Iron is higher that Copper in the table so it displaces Copper from its compounds.
Metals above Hydrogen will displace that substance from acids. Gold is very unreactive - most acids will not attack it. Metals like Zinc and Iron will be dissolved slowly by acids. Magnesium is readilly attacked by acids. Sodium and Potassium are so reactive that they are attacked even by water.
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