Indicators

Indicators are special substances used in chemistry to identify whether a solution is acidic or basic. They are helpful tools that change colour when they come into contact with an acid or a base, making it easy to determine the nature of the substance without tasting or touching it, which can be dangerous.

When we add an indicator to a solution, it reacts in a way that shows a visible change, such as a shift in colour. This change gives us a clue about the pH level of the solution. pH is a scale used to measure how acidic or basic a substance is, ranging from 0 (very acidic) to 14 (very basic), with 7 being neutral.

1. Natural indicator: A natural indicator is a type of indicator that can be found naturally and can determine whether the substance is acidic or basic in nature.

Examples,

• Litmus
• Turmeric
• Red Cabbage Juice
• Onion
• Vanilla

2. Synthetic indicators: A synthetic indicator is a man-made chemical substance used to determine the pH of another substance.

Examples,

• Methyl orange
• Phenolphthalein

3. Universal indicators: A universal indicator is a pH indicator made of a solution of several compounds that exhibits several smooth colour changes over a wide range of pH values to indicate the acidity or alkalinity of solutions.

Indicators in Laboratory;

Laboratory indicators

In the pH scale, ‘p’ stands for ‘potenz’. Potenz is a German word that means ‘power’ or ‘potential’. Here, ‘H’ stands for hydrogen ion. Thus, pH means the potential of hydrogen or the power of hydrogen.

• The strength of an acid or base depends upon the hydrogen ion concentration.
• If the concentration of the hydrogen ion is greater than the hydroxide ion, the solution is called acidic.
• If the concentration of hydrogen ion is smaller than the hydroxide ion, the solution is called basic.
• If the concentration of hydrogen ion is equal to the concentration of hydroxide ion, the solution is called a neutral solution.
• pH is a measure of hydrogen ion concentration, a measure of the acidity or alkalinity of a solution.
• The pH scale usually ranges from 0 to 14. Aqueous solutions at 25 °C with a pH less than 7 are acidic, while those with a pH greater than 7 are basic or alkaline. A pH level of 7.0 at 25 °C is defined as ‘neutral.’

1. Reaction of acids and bases with metals: When an acid reacts with a metal, hydrogen gas is evolved, and a corresponding salt is formed.

Acid + Metal → Salt + Hydrogen gas

2NaOH_(aq) + Zn_(s) → Na₂ZnO₂ _(s) + H₂ _(g)

Examples,

Magnesium reacts with hydrochloric acid, forms magnesium chloride, and liberates hydrogen gas.

Mg + 2HCl → MgCl₂ + H₂

When a base reacts with a metal, hydrogen gas is evolved, and salt is formed, with a negative ion composed of the metal and oxygen.

Examples,

When aluminium reacts with sodium hydroxide, it forms aluminium hydroxide and liberates hydrogen gas.

2Al + 2NaOH + 6H₂O → 2Na⁺ + 2[Al(OH)₄] ^- + 3H₂

2. Reaction of acid with metal carbonate and metal hydrogen carbonates:

Ca(OH)_2(s)+CO_2(g)→CaCO_3(s)+H₂O_(l)

On passing the excess carbon dioxide, we get the calcium bicarbonate.

CaCO₃_(s) + H₂O _(l) + CO₂ _(g) → Ca(HCO₃)₂ _(aq)

When an acid reacts with a metal carbonate or metal hydrogen carbonate, it gives salt, carbon dioxide gas and water.

Metal carbonate/Metal hydrogen carbonate + Acid → Salt + Carbon dioxide + Water

Example,

Calcium carbonate + Sulfuric acid Calcium sulfate + Water + Carbon Dioxide

CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂

3. Reaction of acids with metal oxides:

Metal oxide + Acid → Salt + Water

Metal oxides react with acid to form salt and water.

2HCl + Na₂O → 2NaCl + H₂O

Hydrochloric acid reacts with sodium oxide to form sodium chloride (salt) and H₂O.

4. Reaction of base with non-metal oxides: Non-metallic oxides are acidic in nature; therefore, they react with a base to form salt and water.

Non-metallic Oxide + Base → Salt + Water

Ca(OH)₂_(aq) + CO₂_(g) → CaCO₃_(s) + H₂O_(l)

Calcium hydroxide, when it reacts with carbon dioxide, produces calcium carbonate and water.

5. Reaction of acids and bases:

Acid, when it reacts with a base, produces salt and water

Acid + Base Salt + Water

HCl + NaOH → NaCl + H₂O

When an acid or a base is dissolved in water, they dissociate into ions.

For example,

• When hydrochloric acid is dissolved in water, it gets dissociated into ions such as protons (H⁺ ions) and Cl⁻ ions as follows: HCl + H₂O → H₃O⁺ + Cl⁻
• As there is an increase in the protons in the aqueous solutions, the solution is acidic in nature.
• Similarly, when NaOH is dissolved in water, it gets dissociated as NaOH + H₂O → Na⁺ + OH⁻ + H₂O.
• As there is an increase in the hydroxyl ions in the solution, the solution is basic in nature. A basic aqueous solution is called an alkali.

1. Aim: To observe the reaction of an acid and a base with metals and to identify the gas evolved during the reaction.

2. Requirements: Zinc granules, dilute sulphuric acid (H₂SO₄), test tube, delivery tube, soap solution, candle and matchstick.

3. Procedure:

• Set up the apparatus as shown in the diagram. Place about 5 mL of dilute sulphuric acid in a test tube.
• Add a few pieces of zinc granules to the acid in the test tube.
• Observe the surface of the zinc granules. You will see the formation of bubbles, indicating that gas is being released.
• Connect the test tube to a delivery tube, and pass the gas evolved through a soap solution. The gas will form soap bubbles.
• Take a burning candle near one of the gas-filled soap bubbles.

Observation:

• Bubble formation occurs on the surface of the zinc granules, indicating a chemical reaction.
• The gas passes through the soap solution and forms soap bubbles.
• When a burning candle is brought near the gas-filled bubbles, a pop sound is heard, confirming the presence of hydrogen gas.

Conclusion: When an acid reacts with a metal, hydrogen gas is released, and a salt is formed. The pop sound confirms the presence of hydrogen gas. The chemical reaction for this experiment is:
Acid + Metal → Salt + Hydrogen gas
Example: Zn + H₂SO₄ → ZnSO₄ + H₂

When a base reacts with certain metals, hydrogen gas is also released, forming a salt with a negative ion composed of the metal and oxygen.
Example: 2Al + 2NaOH + 6H₂O → 2Na⁺ + 2[Al(OH)₄]⁻ + 3H₂.

1. Aim: To observe the reaction of an acid with metal carbonate and metal hydrogen carbonate and to identify the gas produced.

2. Requirements: Two test tubes (labeled A and B), sodium carbonate (Na₂CO₃), sodium hydrogen carbonate (NaHCO₃), dilute hydrochloric acid (HCl), and lime water (calcium hydroxide solution)

3. Procedure:

• Take 0.5 g of sodium carbonate (Na₂CO₃) in test tube A.
• Take 0.5 g of sodium hydrogen carbonate (NaHCO₃) in test tube B.
• Add about 2 mL of dilute HCl to both test tubes A and B.
• Observe the gas released in both reactions.
• Pass the gas produced through a lime water solution and observe any changes.

4. Observation:

• When HCl is added to sodium carbonate (test tube A), carbon dioxide gas is released.
• When this gas is passed through lime water, the lime water turns milky white due to the formation of calcium carbonate.
• The chemical reaction for this is:
  Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
• When excess carbon dioxide is passed through the lime water, the milky solution becomes clear due to the formation of calcium bicarbonate:
  CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂

5. Conclusion: When an acid reacts with a metal carbonate or metal hydrogen carbonate, it produces salt, carbon dioxide gas, and water. The reaction confirms that carbon dioxide gas is produced when acids react with these compounds.

General reaction:
Metal carbonate/Metal hydrogen carbonate + Acid → Salt + Carbon dioxide + Water

Examples,
Calcium carbonate + Sulfuric acid → Calcium sulfate + Water + Carbon dioxide
CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂