Notes
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Topics to be learn :
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Acids : Compounds imparting sour taste are called acids.
- Acids are soluble in water and they are corrosive in nature.
- Animals and plants also possess acids in them.
- Acids present in food stuffs are called natural acids or carbonic acids. These acids are weak acids.
- Some acids are strong in nature. They are caustic/inflammatory, e.g., sulphuric acid (H2SO4), hydrochloric acid (HCl) and nitric acid (HNO3). These acids are also called ‘mineral acids’.
Base: Substances having astrigent/bitter taste and slippery to touch are called bases.
- Examples : Lime water Ca(OH)2, baking soda (NaHCO3), caustic soda (NaOH) and soaps.
Indicator : Substances which change their colour in presence of acid or base are called indicators.
Touching or tasting of acid or base is very harmful and hence typical indicators are used to recognise them.
| Name of Indicator | Colour of the Indicator | Colour in Acid | Colour in alkali |
| Litmus paper | Blue | Red | Blue (Remain same) |
| Litmus paper | Red | Red (Remain same) | Blue |
| Methyl orange (Synthetic Indicator) | Orange | Pink | Yellow |
| Phenolphthalein | Colourless | Colourless | Pink |
| Methyl red (Synthetic Indicator) | Red | Red | Yellow |
Vegetables from which natural indicators can be prepared : Natural indicators : Red cabbage, radish, tomato.
Natural acids (organic) : Acids present in foodstuffs are called natural acids.
- Our daily diet contains a few natural (organic) acids.
- They are mild in nature and hence are not harmful like mineral acids.
A few substances having natural acids (organic) are given in the following table :
| Substance / Source | Acids (Natural/organic) |
| Vinegar | Acetic acid |
| Orange | Ascorbic acid |
| Tamarind | Tartaric acid |
| Tomato | Oxalic acid |
| Curd | Lactic acid |
| Lemon | Citric acid |
Properties of acids :
- Acids are sour in taste.
- Acid molecules contain hydrogen ion (H+) as a main constituent.
- Acid reacts with metal to form hydrogen gas.
- Acid reacts with carbonates and liberates CO2 gas.
- Blue litmus turns red in acid.
Use of acids :
- Acids are used in the production of chemical fertilizers.
- Acids are used in the production of explosives, oil purification, medicines, dyes and paints.
- Hydrochloric acid is used for the preparation of different types of chloride salts.
- Dil. H2SO4 acid is used in the batteries (electric cell).
- Dil. HCl is used for sterilization of water.
- Acid is used for making of white paper from wood pulp.
We have seen that mineral acids are harmful to body. However several organic acids are present in our body and in plants which are useful to us.
- DNA (Deoxyribo Nucleic Acid) is an acid present in our body, it decides heredity properties.
- Proteins which are part of our body cell are made up of amino acids.
- Fat of our body is formed by fatty acids.
Q. What happens when sour substances like lime juice, tamarind water falls on shahabad stones or kitchen platform? Why?
When sour substances like lime juice, tamarind water falls on Shahabad stones or kitchen platform, citric acid present in lime juice and tartaric acid in tamarind water reacts with metal carbonates present in shahabad stones or kitchen platform, carbon dioxide gas is evolved resulting in the formation of uneven surface.
Q. Which substances are used to clean greenish stains on copper vessels and to shine blackish silver utensils?
Tamarind pulp is used to clean greenish stains on copper vessels. To shine blackish silver utensils, ammoniacal solution or lime juice or detergent is used.
Q. Why toothpaste is used for brushing teeth?
Base/Alkali : Alkali is a substance whose water solution gives hydroxide (OH-) ion e.g.
NaOH(aq) --------> Na+(aq) + OH+(aq)
(Sodium Hydroxide) (Sodium ion) (Hydroxide ion)
Acid and Alkali :
| Acid | Alkali |
| Acid has sour taste.
Acid turns blue litmus red. An acid in an aqueous solution gives H+ ions. Oxides of non-metals form acids. |
Alkali has bitter taste.
Alkali turns red litmus blue. An alkali in an aqueous solution gives OH- ions. Oxides of metals form bases. |
Neutralization : we have Seen that acid contains (H+) hydrogen ions a alkali contains (OH-) hydroxide ions.
The reaction between an acid and alkali to form the salt and water termed as neutralization.
Acid + Base à Salt + Water
HCl + NaOH ———> NaCl + H2O
Hydrochloric Sodium Sodium Water
acid Hydroxide Chloride
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