Acids, Bases and Salts
Acids, Bases and Salts
This is the most heavily examined topic in the chemistry block, worth 2 questions on its own. Everyday acids, the pH scale and neutralisation in daily life are the three areas that repeat, and all three connect to examples a class 7 student would meet.
Properties and Indicators
| Property | Acids | Bases |
|---|---|---|
| Taste | Sour | Bitter |
| Feel | — | Soapy |
| Blue litmus | Turns red | No change |
| Red litmus | No change | Turns blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Turmeric | No change | Red |
Litmus is obtained from lichens and is a natural indicator, as are turmeric and china rose. Phenolphthalein and methyl orange are synthetic. The lichen origin is asked directly.
✗ Phenolphthalein turns pink in acid | ✓ It stays colourless in acid and turns pink in base
Everyday Acids and Bases
| Acid | Found in | Base | Found in |
|---|---|---|---|
| Citric acid | Lemon, orange | Calcium hydroxide | Lime water |
| Acetic acid | Vinegar | Sodium hydroxide | Soap, drain cleaner |
| Lactic acid | Curd | Magnesium hydroxide | Milk of magnesia |
| Oxalic acid | Tomato, spinach | Ammonium hydroxide | Window cleaner |
| Formic acid | Ant and bee sting | — | — |
| Tartaric acid | Tamarind | — | — |
| Hydrochloric acid | Stomach (digestion) | — | — |
The acid column is asked more often than any other table in the chemistry block. Formic acid in an ant sting is the single most repeated pair, because it comes with a neutralisation remedy attached.
The pH Scale
pH 0 to 7 acidic | pH 7 neutral | pH 7 to 14 basic
- A lower pH means a stronger acid; distilled water has a pH of exactly 7.
- Human blood is slightly basic, around 7.4.
- Soil pH decides which crops will grow; farmers add lime to acidic soil.
- Tooth decay begins when the mouth's pH falls below about 5.5, which is why toothpaste is basic.
The tooth-decay example is a favourite because it ties the pH scale to a daily habit, and it is exactly the kind of application a teaching paper prefers over abstract chemistry.
Neutralisation in Daily Life
Acid + Base → Salt + Water
| Situation | Problem | Remedy |
|---|---|---|
| Indigestion | Excess stomach acid | Antacid such as milk of magnesia |
| Ant sting | Formic acid | Baking soda or calamine |
| Acidic soil | Too low a pH | Quicklime or slaked lime |
| Basic soil | Too high a pH | Organic manure |
| Factory waste | Acidic effluent | Neutralised before release |
Neutralisation is exothermic, meaning heat is released. Salts themselves may be acidic, basic or neutral depending on which acid and base formed them — common salt is neutral, which is a small but examinable point.
Acid rain is caused by sulphur and nitrogen oxides from burning fuels dissolving in rainwater. It damages crops, buildings and aquatic life, and it links this topic to the environment questions in Part-II.
pH 7 उदासीन, 7 से कम अम्लीय, 7 से ज्यादा क्षारीय।
TRE pointer: Two guaranteed items: the acid-and-source table, and a neutralisation-in-daily-life question, usually the ant sting or the antacid. The tooth-decay pH threshold is the most current-feeling detail and it appears more often than candidates expect. Note that turmeric turning red in a base is asked because it is a home experiment a teacher can demonstrate. With five options and −1/3 negative marking, this table repays memorising outright.
60-Second Recap
- Acids turn blue litmus red; bases turn red litmus blue and feel soapy.
- Phenolphthalein is colourless in acid and pink in base; turmeric turns red in base.
- Litmus comes from lichens; turmeric and china rose are also natural indicators.
- Citric in lemon, acetic in vinegar, lactic in curd, oxalic in tomato, formic in an ant sting.
- pH below 7 acidic, exactly 7 neutral, above 7 basic; tooth decay begins below about 5.5.
- Acid plus base gives salt and water, and the reaction releases heat.
Frequently Asked Questions
Which acid is present in an ant sting and how is it treated?
Formic acid. It is neutralised by rubbing on a mild base such as baking soda or calamine solution, which contains zinc carbonate. This pairing of acid and remedy is the most frequently asked neutralisation example in teaching examinations.
What does a pH of 7 mean?
It means the solution is neutral, like distilled water. Values below seven indicate acidity and values above seven indicate a basic or alkaline solution, on a scale running from zero to fourteen. Treating seven as acidic is the most common error in this topic.
Why is toothpaste basic?
Because bacteria in the mouth produce acid after eating, and tooth decay begins when the pH falls below about five point five. A basic toothpaste neutralises that acid and protects the enamel. This everyday application is a favourite question in teaching papers.
Where does litmus come from?
From lichens, which are organisms growing on rocks and tree bark. Litmus is therefore a natural indicator, as are turmeric and china rose petals. Phenolphthalein and methyl orange, by contrast, are synthetic indicators prepared in the laboratory.