Ruhaan could tell lemon juice and baking soda were different — one was sharp and sour, one tasted bitter and felt slippery. But how does science explain that difference? And how do we test it without just tasting? This post covers every difference between acids and bases — the kind that shows up in 3-mark and 5-mark exam questions.
This is the comparison table to know for your exam. Every row is a potential exam question.
| Property | Acid | Base |
|---|---|---|
| Taste | Sour | Bitter |
| Feel | No special feel | Soapy / slippery |
| Effect on blue litmus | Turns red | No change |
| Effect on red litmus | No change | Turns blue |
| Effect on turmeric | No change (stays yellow) | Turns red |
| Effect on China rose | Dark pink / magenta | Turns green |
| Nature | Acidic | Basic / alkaline |
| Common examples | Lemon juice, vinegar, curd, tamarind | Baking soda, soap, lime water, washing soda |
| Reaction with each other | Acid + Base → Salt + Water (Neutralisation) | |
The litmus test is the standard method for identifying acids and bases in Class 7 Science. Litmus is a natural dye extracted from lichens. It comes in two forms:
Q: How can you use litmus paper to identify whether a solution is acidic or basic?
A: Dip a piece of blue litmus paper into the solution. If it turns red, the solution is acidic. If there is no change, it may be basic or neutral. Then dip a piece of red litmus paper. If it turns blue, the solution is basic. If neither paper changes colour, the solution is neutral.
"If blue litmus doesn't change, the solution must be neutral." — This is not necessarily true. Blue litmus not changing only tells you the solution is not acidic. It could still be basic or neutral. You need to test with red litmus as well to be sure.
| Substance | Acid / Base / Neutral | Acid/Base present |
|---|---|---|
| Lemon juice | Acid | Citric acid |
| Vinegar | Acid | Acetic acid |
| Baking soda | Base | Sodium bicarbonate |
| Soap | Base | Sodium hydroxide |
| Pure water | Neutral | — |
When an acid and a base come together, they react in a process called neutralisation. They cancel each other out and form two new substances:
Neutralisation reaction: Acid + Base → Salt + Water
Example: Hydrochloric acid + Sodium hydroxide → Sodium chloride (table salt) + Water
After neutralisation, the solution is neither acidic nor basic — it is neutral (if exact amounts are used).
A neutral substance is one that is neither acidic nor basic. It does not change the colour of blue or red litmus paper. The most common example of a neutral substance is pure water. Table salt dissolved in water is also approximately neutral.
✅ 3-mark answer
Q: State three differences between acids and bases.
A:
1. Taste: Acids taste sour. Bases taste bitter.
2. Effect on litmus: Acids turn blue litmus red. Bases turn red litmus blue.
3. Feel: Acids have no special feel. Bases feel soapy or slippery to touch.
Acids taste sour, turn blue litmus red, and are found in substances like lemon juice and vinegar. Bases taste bitter, feel soapy, turn red litmus blue, and are found in substances like baking soda and soap. They react with each other to form salt and water in a neutralisation reaction.
A neutralisation reaction occurs. The acid and base react to form salt and water. The resulting solution is neutral. General equation: Acid + Base → Salt + Water.
Blue litmus paper turns red in acids and stays unchanged in bases. Red litmus paper turns blue in bases and stays unchanged in acids. If neither paper changes colour, the solution is neutral.
A neutral substance is neither acidic nor basic. It does not change the colour of blue or red litmus paper. Pure water is the most common example of a neutral substance.
No. Turmeric does not change colour in acidic solutions — it stays yellow. Turmeric turns red only in basic (alkaline) solutions. This is why a turmeric stain turns red when you apply soap (a base) to it.