To test whether something is acidic or alkaline, we can use either a litmus paper or _______
turmeric
Testing Acidic and Alkaline Substances
To find out if a substance is acidic or alkaline (basic), we use special materials called indicators. These indicators change colour when they come into contact with an acidic or alkaline substance. Litmus paper is a very common indicator used for this purpose.
How Litmus Paper Works
Litmus paper is available as red and blue strips. It changes colour depending on whether the substance is acidic, alkaline, or neutral:
- If a substance is acidic, it turns blue litmus paper red.
- If a substance is alkaline, it turns red litmus paper blue.
- If a substance is neutral, neither red nor blue litmus paper changes colour.
Using Turmeric as a Natural Indicator
Besides litmus paper, many other substances can act as indicators. Turmeric is a common natural indicator. Turmeric is a spice that is bright yellow in colour.
- When turmeric comes into contact with an acidic substance, its yellow colour usually stays the same or changes only slightly.
- However, when turmeric comes into contact with an alkaline substance, its yellow colour changes to red or reddish-brown. This colour change is very noticeable and is often seen when turmeric stain on clothes is washed with soap (which is alkaline).
- Neutral substances do not cause a significant colour change in turmeric.
Therefore, turmeric can be used to test whether a substance is acidic or alkaline, similar to how litmus paper is used.
Why Other Options Are Not Indicators
Let's look at the other options provided:
- Soda ash: Soda ash is sodium carbonate (Na<sub>2</sub>CO<sub>3</sub>). It is an alkaline substance itself, not an indicator used to test other substances.
- Sugar: Sugar (sucrose) is a neutral substance and does not change colour in response to acids or alkalis. It is not an indicator.
- Salt: Common salt (sodium chloride, NaCl) is usually a neutral substance. It also does not change colour when mixed with acids or alkalis and is not used as an indicator.
Based on the properties of these substances, only turmeric functions as an indicator among the given options, similar to litmus paper.
Revision Table: Common Indicators
| Indicator | Colour in Acidic Solution | Colour in Alkaline Solution | Colour in Neutral Solution |
|---|---|---|---|
| Litmus Paper (Blue) | Red | No Change (Stays Blue) | No Change (Stays Blue) |
| Litmus Paper (Red) | No Change (Stays Red) | Blue | No Change (Stays Red) |
| Turmeric | Yellow | Red / Reddish-brown | Yellow |
| Phenolphthalein | Colourless | Pink | Colourless |
| Methyl Orange | Red | Yellow | Orange |
Additional Information on pH Scale and Indicators
Acidity and alkalinity are measured using the pH scale. The pH scale ranges from 0 to 14.
- Substances with a pH less than 7 are acidic.
- Substances with a pH equal to 7 are neutral.
- Substances with a pH greater than 7 are alkaline (or basic).
Indicators like litmus and turmeric help us determine if a substance is acidic or alkaline by showing different colours at different pH ranges. They are simple tools for quick identification. More complex indicators or pH meters can give a more precise pH value.
Natural indicators are derived from plants. Besides turmeric, other natural indicators include red cabbage juice, grape juice, and petal extracts of flowers like hibiscus or petunia.
Important Questions from Equilibrium
A stain of curry on a white cloth become reddish brown when soap is used because the soap is:
Bitter tasting food is ______ in nature.
Which of the following statements is true?
In a neutralisation reaction, a new substance is formed, called _____.
Acidic and basic solutions in water conduct electricity because they produce ______ and hydroxide ion, respectively.
