Properties of Acids

INDICATORS:-

Natural indicator:-

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

Synthetic indicators:-

Synthetic indicator is a man made chemical substance that is used to determine the pH of another substance.

Examples:-

• Methyl orange
• Phenolphthalein

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 pH values to indicate the acidity or alkalinity of solutions.

CONCEPT OF pH SCALE:-

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

The strength of acid or base depends upon the hydrogen ion concentration. If the concentration of hydrogen ion is greater than 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 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 solution 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’.

REACTION OF ACID & BASES WITH METALS:-

When an acid reacts with a metal, hydrogen gas is evolved and a corresponding salt is formed.

Given below is the reaction of Zinc Granules with Dilute sulphuric acid

• Set the apparatus as shown in Fig

• Take about 5 mL of dilute sulphuric acid in a test tube and add a few pieces of zinc granules to it.

• What do you observe on the surface of zinc granules?

• Pass the gas being evolved through the soap solution.

• Why are bubbles formed in the soap solution?

• Take a burning candle near a gas filled bubble.

Observation:-

• Bubble formation takes place inside the test tube

• The gas passes through the delivery tube into the soap solution resulting in formation of soap bubbles

• when a burning candle is taken near the gas filled bubble we hear a pop sound, this is because the soap bubbles are filled with hydrogen gas.

Acid + Metal → Salt + Hydrogen gas

2NaOH_(aq) + Zn_(s) → Na₂ZnO_2(s) + H_2(g)

Examples:-

When Magnesium reacts with Hydrochloric acid it 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 which has 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

REACTION OF ACID WITH METAL CARBONATE AND METAL HYDROGEN CARBONATES:-

• Take two test tubes, label them as A and B.
• Take about 0.5 g of sodium carbonate (Na₂CO₃) in test tube A and about 0.5 g of sodium hydrogencarbonate (NaHCO₃) in test tube B.
• Add about 2 mL of dilute HCl to both the test tubes
• Pass the gas produced in each case through lime water (calcium hydroxide solution)

OBSERVATION:-

• When HCL is added to sodium carbonate carbon dioxide gas is liberated.
• When this gas is passed through lime water a white precipitate of calcium carbonate is formed
• When we pass carbon dioxide gas through lime water then calcium carbonate is formed.

Ca(OH)_2(s)+CO_2(g)→CaCO_3(s)+H₂O_(l)

On passing the excess carbon dioxide we get the calcium bicarbonate

CaCO_3(s) + H₂O_(l) + CO_2(g) → Ca(HCO₃)_2(aq)

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

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

Example:-

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

CaCO₃+ H₂SO₄ → CaSO₄+ H₂O + CO₂

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

REACTION OF BASE WITH NON METAL OXIDES:-

Non metallic oxides are acidic in nature therefore react with base to form salt and water.

Non-metallic Oxide + Base → Salt + Water

Ca(OH)_2(aq) + CO_2(g) → CaCO_3(s) + H₂O_(l)

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

REACTION OF ACID AND BASES:-

Acid when react 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 get dissociated into ions.

For example,

• When hydrochloric acid is dissolved in water, it get 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 get 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.
• Basic aqueous solution is called alkali.