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Science: Acids, Bases and Salts: Properties of Acids and Bases
Acid
- An acid is a substance which furnishes hydrogen ions (H+ ) when dissolved in water.
- For example, in its aqueous solution hydrochloric HCl (aq) dissociates as:
HCl (aq) ⇾ H+ (aq) + Cl– (aq)
e. g. Hydrochloric acid (HCl) in gastric juice, Carbonic acid (H2CO3) in soft drinks.
Bases
- A base is a substance which furnishes hydroxide ions (OH– ) when dissolved in water
- For example, sodium hydroxide NaOH (aq) , in its aqueous solutions, dissociates as:
NaOH (aq) ⇾ Na+ (aq) + OH– (aq)
e. g. Sodium hydroxide (NaOH) or caustic soda used in washing soaps.
Potassium hydroxide (KOH) or potash used in bathing soaps.
Indicators
Turmeric has acted as an indicator of base present in soap. There are many substances that show one colour in an acidic medium and another colour in a basic medium. Such substances are called acid-base indicators. Litmus is a natural dye found in certain lichen . shows red colour in acidic solutions and blue colour in basic solutions. Phenolphthalein and methyl are some indicators.
Properties of Acids
Taste
Acids have a sour taste.
Action on Indicators
Indicator | Colour in acidic medium | |
---|---|---|
1. | Litmus | Red |
2. | Phenolphthalein | Colorless |
3. | Methyl orange | Red |
Conduction of Electricity and Dissociation of Acids
When acids are dissolved in water, they produce ions which help in conducting the electricity. This process is known as dissociation
Reactions with Acids
Reaction of Acid with Metal
Acid + Metal ⇾ Salt + Hydrogen gas
Mg + H2 SO4 ⇾ H2 + Mg SO4
Reaction of Acid with Carbonates
Na2 CO3 (s) + 2 HCl (aq) ⇾ 2NaCl (aq) + H2O (l) + CO2 (g)
Reaction of Acid with Bicarbonates
NaHCO3 (s) + HCl (aq) ⇾ NaCl (aq) + H2O (l) + CO2 (g)
Properties of Bases
- Aqueous solutions of bases are also electrolytes. Bases can be either strong or weak, just as acids can.
- Bases often have a bitter taste and are found in foods less frequently than acids. Many bases, like soaps, are slippery to the touch.
- Bases also change the color of indicators. Litmus turns blue in the presence of a base while phenolphthalein turns pink.
- Bases do not react with metals in the way that acids do.
- Bases react with acids to produce a salt and water.
- Reactions of Bases
- Reaction with Metals
Base reacts with metals and produce hydrogen gas.
2NaOH + Zn ⇾ Na2 ⇾ Na2ZnO2 + H2
Reaction with Acids
Base reacts with acids to form salts.
For Example,
KOH + HCl ⇾ KCl + H2O
Reaction with Non-Metallic Oxides
Base reacts with non-metallic oxides to form salt and water.
2NaOH + CO2 ⇾ CO2 ⇾ Na2CO3 + H2O