Physical and Chemical Properties of Acids, Preparation, Test and Uses of Acids.

Physical Properties of Acids

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1. Dilute acids have a sour taste. For example, the sour taste of unripe fruits, vinegar and rancid milk is due to the presence of acids in them. (Note: Never taste laboratory acids).

2. Acids turn blue litmus to red. They turn methyl orange from orange/yellow to pink or red. Phenolphthalein remains colorless.

3. The concentrated forms of strong acids like hydrochloric acid, trioxonitrate(V) acid and tetraoxosulphate(VI) acid are corrosive. In this regard, they cause severe burns or damage to skin and other materials.

4. Most acids are liquids at room temperature (e.g. HCl, H₂SO₄, HNO₃, CH₃COOH), though some exist as solids (e.g. citric acid, tartaric acid, solid ethanedioic/oxalic acid), or as gases (that form aqueous acidic solution when dissolved in water).

5. Aqueous solutions of acids are electrolytes, and they conduct electricity because they ionize in water to produce mobile ions. However, note that strong acids conduct better than weak acids of the same concentration, since they have more free ions.

6. Acids generally have a pH less than 7. The lower the pH, the more strongly acidic the substance is.

7. Concentrated acids like HCl and HNO₃ are fuming. In this regard, they give off visible acidic vapor in moist air.

8. Most common acids are freely soluble in water (with dilution often releasing/absorbing heat, sometimes violently). In this regard, always add acid to water, and never water to acid, to avoid dangerous spattering.

9. Concentrated sulfuric acid is an oily, dense liquid and a powerful dehydrating agent (removes water from substances). It can char (blacken) organic materials such as sugar or paper.

Please read an introduction to acids here.

 

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Chemical Properties of Acids

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1. Reaction with metals

Acids react with metals metals that are more reactive than hydrogen (that is; above hydrogen in the reactivity series); e.g zinc (Zn), iron (Fe) and magnesium (Mg) to liberate hydrogen gas, while also forming a salt. This reaction proceeds as a result of the displacement of hydrogen ions in the acids by the metals. However, understand that dilute trioxonitrate(V) acid (HNO₃) is an exception to this rule since it is a powerful oxidizing agent, and does not liberate hydrogen gas from most metals, with the exception of magnesium (Mg) and manganese (Mn) (which reacts with very dilute and cold nitric acid).

Below are equations that show the reaction of acids with metals.

• 2HCl_(aq) + Zn_(s) -> ZnCl_2(s) + H_2(g)

• 2HCl_(aq) + 2Li_(s) -> 2LiCl_(s) + H_2(g)

• H₂SO_4(aq) + Fe_(s) -> FeSO_4(s) + H_2(g)

• H₂SO_4(aq) + 2Na_(s) -> Na₂SO_4(aq) + H_2(g)

Note: Unreactive metals like copper, silver, and gold do not react with dilute acids in this way. It will rather produce oxides of nitrogen instead of H₂, due to its oxidizing nature.

You can read on families of elements in the periodic table here.

 

2. Reaction with bases and alkalis

Acids react with insoluble bases (metal oxides, hydroxides) and alkalis to form salt and water as the only product. Such reaction is termed a neutralization reaction.

• Acid + Base (or Alkali) -> salt + water

Note: An alkali is a soluble base. Examples of alkali are sodium hydroxide (NaOH) and potassium hydroxide (KOH).

Below are equations that show neutralization reaction:

• H₂SO_4(aq) + CaO_{(s) ->} CaSO_4(aq) + H₂O_(l)

• HCl_(aq) + NaOH_(aq) -> NaCl_(aq) + H₂O_(l)

• 2HNO_3(aq) + Ca(OH)_2(aq) -> Ca(NO₃)_2(aq) + 2H₂O_(l)

• H₂SO_4(aq) + 2KOH_(aq) -> K₂{SO}_4(aq) + 2H₂O_(l)

• 2HCl_(aq) + CuO_(s) → CuCl_2(aq) + H₂O_(l)

Please read on salts in chemistry here.

 

3. Reaction with trioxocarbonates(IV)

Acids react with trioxocarbonates(IV) or metal carbonates or hydrogen trioxocarbonate(IV) to produce salt, water and the liberation of carbon(IV)oxide gas. This is expressed below:

• Acid + Trioxocarbonate(IV) -> Salt + Water + Carbon(IV)oxide

Below are equations that show the reaction of acids with trioxocarbonates(IV)

• 2HCl(aq) + MgCO_2(aq) -> MgCl_2(aq) + H₂O_(l) + CO_2(g)

• H₂SO_4(aq) + K₂CO3(aq) -> K₂SO_4(aq) + H₂O_(l) + CO_2(g)

• CuCO_3(s) + H₂SO_4(aq) -> CuSO₄ + H₂O_(l) + CO_2(g)

• 2HNO₍₃₎ + Na₂CO₃ -> 2NaNO_3(aq) + H₂O_(l) + CO_2(g)

• H₂SO_4(aq) + 2NaHCO_3(aq) -> Na₂SO_4(aq) + H₂O_(l) + CO_2(g)

Note: Acids also reacts with bicarbonates (or hydrotrioxocarbonate IV) to form salt, water and carbon(IV)oxide. This is shown via the equations below:

• HCl_(aq) + NaHCO_3(s) -> NaCl_(aq) + H₂O_(l) + CO_2(g)

• H₂SO_4(aq) + 2NaHCO_3(aq) -> Na₂SO_4(aq) + 2H₂O + 2CO_2(g)

You can read on mole and Avogadro's number in chemistry here.

 

4. Oxidizing properties (Conc. Acids)

Concentrated sulfuric (H₂SO₄) and nitric acids (HNO₃) act as oxidizing agents, especially when hot, reacting with metals (even some below hydrogen in the reactivity series, like copper) without liberating hydrogen gas (H₂). This is shown via the equations below:

• Cu_(s) + 2H₂SO_4(aq)(conc.) --(heat)--> CuSO_4(aq) + SO_2(g) + 2H₂O_(l)

• Cu_(s) + 4HNO₃(conc.) -> Cu(NO₃)_2(aq) + 2NO_2(g) + 2H₂O_(l)

 

5. Dehydrating property of conc. H2SO4

Concentrated H₂SO₄ is an hygroscopic agent that removes water (or the elements of water) from compounds. This is shown via the equation below (where it blackens or chars sugar to black carbon). 

C₁₂H₂₂O₁₁ --(conc. H₂SO₄) -> 12C_(s) + 11H₂O_(l)

 

6. Effect on indicators

Acids change the color of acid-base indicators. This is further explained in the latter part of this article. However, it is summarized via the table below:

Indicator	Color in Acid
Blue litmus	Turns red
Red litmus	Stays red (no change)
Methyl orange	Red/pink
Phenolphthalein	Colorless
Bromothymol blue	Turns yellow
Universal indicator	Red to yellow (pH < 7)

You can read on the postulate of the kinetic theory of gases here.

 

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Laboratory Preparation of Acids

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Acids can be prepared in the laboratory via a variety of ways. They include the following:

1. Dissolving acid anhydride in water

Acid anhydrides are oxides of nonmetals which are acidic in nature. In this regard, they react with water to form acids. Examples of acidic oxides include carbon(IV)oxide (CO₂), sulphur(IV)oxide (SO₂), nitrogen(IV)oxide (NO₂),  sulfur trioxide (SO₃), dinitrogen pentoxide (N₂O₅), phosphorus pentoxide (P₄O₁₀) or (P₂O₅).

Below are equations that shows the formation of acids when acid anhydride is dissolved in water.

• CO_2(g) + H₂O_(l) -> H₂CO_3(aq)

• SO_3(g) + H₂O_(l) -> H₂SO_4(aq)

• N₂O_5(s) + H₂O_(l) -> 2HNO_3(aq)

• 4P₄O_10(s) + H₂O_(l) -> H₃PO_4(aq)

• SO_2(g) + H₂O_(l) -> H₂SO_3(aq)

You can attempt various questions with answers in chemistry here.

 

2. Combination of constituent elements

Certain nonmetals reacts directly with hydrogen under specific conditions (such as heat, sunlight or electric spark) to produce an acidic gas which is dissolved in water to form acids.

Below are equations that shows the formation of acids through direct combination of elements.

• H₂ + Cl₂ -> 2HCl_(g)

In the above equation, hydrogen gas reacts with chlorine gas under the influence of sunlight or electric spark to produce hydrogen chloride gas; which can be subsequently dissolved in water to produce hydrochloric acid.

• H_2(g) + S_(s) -> H₂S_(g)

In the above reaction, hydrogen gas combines directly with solid sulphur under the influence of heat to produce hydrogen sulphide.

• H_2(g) + Br_2(g) -> 2HBr_(g)

In the above reaction, hydrogen gas reacts with bromine vapor in the presence of heat, sunlight or finely divided platinum catalyst at elevated temperatures to produce hydrogen bromide gas, which dissolves readily in water to form hydrobromic acid.

You can read Dalton's atomic theory and its modifications here.

 

3. By using a strong acid to displace a weak acid or a volatile acid from its salt.

Acids are produced when a strong acid reacts with the salt of a weaker or volatile acid. This procedure of producing acid is often done with gentle heating, and is commonly used to make hydrogen chloride or nitric acid in the laboratory. This is shown via the equations below:

• NaCl_(s) + H₂SO_4(aq) conc --(heat) -> NaHSO_4(s) + HCl_(g)

The HCl gas produced (above) is then dissolved in water to give hydrochloric acid.

• NaNO₃ + H₂SO₄ (conc.) --(gentle heat) -> NaHSO_4(s) + HNO_3(g)

HNO₃ vapor (above) is condensed/collected to give nitric acid.

 

Below are more equations where a stronger acid is used to displace a weaker acid or more volatile acid from its salt:

• 2NaNO_3(s) + H₂SO_4(aq) -> Na₂SO_4(aq) + 2HNO_3(g)

In the above reaction, concentrated tetraoxosulphate(VI) acid displaces the more volatile trioxonitrate(V) acid from its trioxonitrate(V) salt, that is (NaNO₃).

• 2KCl_(s) + H₂SO_4(aq) -> K₂SO_4(aq) + 2HCl_(g)

In the above reaction, concentrated tetraoxosulphate(VI) acid displaces the more volatile hydrochloric acid from its potassium chloride salt, that is (KCl)

• CH₃COONa_(s)(s) + H₂SO₄ -> NaHSO_4(aq) + CH₃COOH_(aq)

In the above reaction, concentrated tetraoxosulphate(VI) acid displaces the more volatile ethanoic acid (or acetic acid) from its sodium ethanoate (or sodium acetate) salt, that is (CH₃COONa)

Please read on the differences between physical and chemical change in chemistry here.

 

4. By fermentation (for some organic acids)

Ethanoic acid can be produced by the oxidation or fermentation of ethanol (using a bacteria named Acetobacter, in the presence of air). This is how vinegar is traditionally made; and is shown via the equation below:

• C₂H₅OH_(aq) + O_2(g) --(Acetobacter) -> CH₃COOH_(aq) + H₂O_(l)

 

Note: Acids may also be manufactured at an Industrial-scale. This is explained briefly below:

• Sulfuric Acid (H₂SO₄): It is manufactured by the contact process; which involves the burning sulfur/roasting sulfide ores to SO_2, followed by its catalytic oxidation to SO₃ using V₂O₅ catalyst, then absorption of SO₃ to form oleum, which is diluted to give H₂SO₄.

• Nitric Acid (HNO₃). It is manufactured by the Ostwald process, which involves the catalytic oxidation of ammonia to NO, with further oxidation to NO₂, then absorption in water to give HNO₃.

• Hydrochloric Acid (HCl): It is manufactured by dissolving hydrogen chloride gas, (a by-product of other industrial processes such as organic chlorinations).

You can read on homogenous and heterogenous mixtures in chemistry here.

 

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Test for Acids

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A substance can be tested for its acidity via the following ways.

 

1. Litmus paper test

Test: Dip blue litmus paper into the substance or solution.

Observation: Blue litmus paper turns red in the presence of an acid.

Note: A red litmus paper stays red when dipped in an acid. 

2. Universal indicator / pH test

Test: Add a few drops of universal indicator solution (or dip in universal indicator paper) to the sample of acid substance or solution.

Observation: The color obtained corresponds to a pH value less than 7 (typically red/orange/yellow) for acids on the standard color chart, with pH 1 being strongest.

 

3. Reaction with a reactive metal

Test: Add a small piece of reactive metal (e.g. magnesium ribbon) to the suspected acid.

•  Mg_(s) + 2HCl_(aq) → MgCl_2(aq) + H_2(g)

Observation: Effervescence (that is, bubbling) occurs, and the gas produced (hydrogen) gives a pop sound when tested with a burning or lit splint.

 

4. Trioxocarbonate and bicarbonate test

The addition of an acid to a trioxocarbonate (e.g Na₂CO_3, CaCO_3, MgCO₃) or a bicarbonate (NaHCO₃) give rise to the formation of a salt, water and carbon(IV)oxide gas. This release of CO₂ gas is seen as a visible fizzing or bubbling, confirming that an acid had been added.

Test: Add the suspected acid to a carbonate or bicarbonate, for example, Na₂CO_3, CaCO₃, or NaHCO₃. This is shown via the equations below:

• 2HCl_(aq) + Na₂CO_3(aq) -> 2NaCl_(aq) + H2O_(l) + CO_2(g)

• CaCO_3(g) + 2HCl_(aq) → CaCl_2(aq) + H₂O_(l) + CO_2(g)

• H₂SO_4(aq) + 2NaHCO_3(s) -> Na₂SO_4(aq) + 2H₂O_(l) + 2CO_2(g)

Observation: Effervescence occurs, releasing a colorless gas that turns lime water milky. This colorless gas is CO₂.

You can read on mixtures and characteristics of mixtures here.

 

5. pH paper or strips

An acid turns a universal pH paper red, orange, or yellow, depending on its level.of acidity. The pH values ranges from 0 (strongly acidic) to less than 7 (weakly acidic).

 

6. Liquid Indicators

These indicators include methyl orange, phenolphthalein and bromothymol blue. The methyl orange indicator turns red in an acidic medium while phenolphthalein remains colorless in an acidic medium. Bromothymol blue turns yellow in an acidic medium.

The table below shows the color change summary of acids in indicators:

Indicator	Color in Acid
Blue litmus	Turns red
Red litmus	Stays red (no change)
Methyl orange	Red/pink
Phenolphthalein	Colorless
Bromothymol blue	Turns yellow
Universal indicator	Red to yellow (pH < 7)

 

You can read on physical and chemical change in chemistry here.

 

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Uses of Acids

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1. Sulfuric acid (H₂SO₄)

H₂SO₄ is the most used chemical in the world. It serves the following purposes:

• In theanufacture of fertilizers (e.g. superphosphate, ammonium sulfate).

• Electrolyte in lead-acid car batteries.

• Manufacture of detergents, paints, dyes, and explosives.

• Petroleum refining and metal processing (e.g. pickling steel to remove rust/scale).

• As a dehydrating agent and drying agent in the laboratory and industry.

 

2. Hydrochloric acid (HCl)

• It is present in gastric juic. It aids digestion of food in the stomach. It can also kill ingested germs in food.

• It is used industrially for pickling; that is, cleaning or descaling metal surfaces before galvanizing/plating.

• It is used in the production of chlorides and dyes. It is also used in food processing (e.g. producuction of corn syrup).

• It is used in the leather tanning industry.

• It is also used to clean concrete, bricks, and swimming pools.

 

3. Nitric acid (HNO3)

• It is used in the manufacture of fertilizers (e.g. ammonium nitrate).

• Manufacture of explosives (e.g. TNT, nitroglycerin).

• Manufacture of dyes, drugs, and plastics.

• It is used as a powerful oxidizing agent in metal etching (processing).

You can read on ionic theory and electrolysis here.

 

4. Phosphoric(V) acid (H3PO4)

• It is used in the manufacture of phosphate fertilizers.

• It is used as a flavoring or acidifying agent in some soft drinks (e.g. colas).

• It is used as a rust remover in metal surface treatment (phosphating, before painting).

 

5. Ethanoic (acetic) acid (CH3COOH)

• Ethanoic acid is the main chemical compound that makes up vinegar. As vinegar, it is used as a food preservative, flavoring, and in pickling.

• It is used in the manufacture of synthetic fibers (e.g. cellulose acetate/rayon), plastics, and solvents.

• It is also used in dyeing silk and other protein or polyamide textiles.

 

6. Citric acid

• It is used as a food preservative and flavoring agent (souring agent) in foods and beverages.

• It is used in cleaning products and effervescent tablets.

 

7. Lactic acid

• It is used in food preservation (fermented foods, yoghurt, sourdough).

• It is used in the leather/textile industry.

You can read on the physical properties of metals here.

 

8. Carbonic acid (H2CO3)

• It is present in carbonated (fizzy) drinks, giving them their characteristic sharp taste and fizz.

 

9. Boric acid (H3BO3)

• It is used as a mild antiseptic or germicide for killing pathogenic microorganisms.

 

10. Tartaric acid (C4H6O6)

• It is used in making baking soda, soft drink and health salts.

 

11. Citric acid (C6H8O7)

• It is present in fruits, and is applied in the manufacture of fruit juice.

You can read calculation questions on Charles' law in chemistry here

 

12. Fatty acids

Fatty acids like palmitic and stearic acids are used in the manufacture of soap through a process termed saponification. The chemical process is shown below.

• Fatty acid + Caustic Soda -> Soap + Water.

 

Other uses of acids include:

• Acids are used extensively as catalysts and reagents in the chemical and pharmaceutical industries.

• Acids are used in analytical chemistry for titrations and other quantitative analyses.

• Acids are used in cleaning and descaling applications (e.g. removing limescale, rust).

• Acids are used in electroplating and metal finishing processes.

Please read on the continuation of this topic here (Bases)