Questions & Answers: "Exploring Substances - Acidic, Basic and Neutral"

Blue litmus paper turns red in acidic solutions. Lemon juice is an acidic substance due to the presence of citric acid. Baking soda solution and soap solution are basic in nature and would turn red litmus blue (or have no effect on blue litmus). Sugar solution is neutral and would not change the color of blue litmus paper.

Turmeric is a natural indicator. It remains yellow in acidic and neutral solutions but turns reddish-brown or red in basic solutions. Since the turmeric paper changed to reddish-brown, the unknown liquid must be basic in nature.

This process is known as neutralization. When an acid reacts with a base, they neutralize each other's effects, and the products formed are salt and water. If the acidic and basic solutions are mixed in appropriate amounts, the resulting solution will be neutral, meaning it is neither acidic nor basic.

Basic substances are typically bitter in taste and feel soapy or slippery to touch. Acids are sour in taste and turn blue litmus red. While some bases can react with certain metals, feeling soapy to touch is a more general and distinct characteristic mentioned for bases in Class 7 textbooks, along with their bitter taste.

Distilled water is considered a neutral substance, meaning it is neither acidic nor basic. Vinegar (acetic acid) and curd (lactic acid) are acidic. Washing soda solution (sodium carbonate) is basic in nature. Therefore, distilled water is the correct neutral substance among the given options.

Lemon juice is an acidic substance. Turmeric acts as a natural indicator. Turmeric solution remains yellow when an acidic substance is added to it. It changes to reddish-brown only when a basic substance is added. Therefore, adding lemon juice to a turmeric solution will not change its yellow color.

Acids are substances that turn blue litmus paper red. Lemon juice is a well-known acidic substance due to the presence of citric acid. Baking soda solution, soap solution, and lime water are all basic substances, which would turn red litmus paper blue, or have no effect on blue litmus.

Bases are substances that turn red litmus paper blue. Detergent solution is a common example of a basic solution. Vinegar and curd are acidic substances, which would turn blue litmus red. Sugar solution is a neutral substance and would not change the color of either red or blue litmus paper.

Turmeric is a natural indicator that remains yellow in acidic and neutral solutions but turns red (or reddish-brown) in basic solutions. Soap solution is basic in nature, so when turmeric solution is added to it, the color will change from yellow to red.

China rose indicator (Hibiscus flower extract) is another natural indicator. It turns dark pink or magenta in acidic solutions and green in basic solutions. It has no significant color change in neutral solutions.

To neutralize acidic soil, a basic substance needs to be added. Slaked lime, which is calcium hydroxide, is a basic compound commonly used to reduce the acidity of soil. Vinegar and lemon juice are acidic and would increase the soil's acidity. Common salt is a neutral substance and would not effectively neutralize the acidity.

Turmeric is a natural indicator that turns red or reddish-brown in basic solutions. In acidic or neutral solutions, it remains yellow.

The reaction between an acid and a base is called neutralization, which produces salt and water. However, the resulting solution is only neutral if the acid and base are of appropriate strengths and taken in suitable amounts. If one is stronger or in greater quantity, the resulting solution can still be acidic or basic.

While some food items like lemon juice contain mild acids and are safe to consume, many acids, especially strong ones used in laboratories, are corrosive and dangerous. It is unsafe and highly discouraged to taste or touch unknown substances.

Grading Rubric: 1. Correct colour change for basic solutions (yellow to red/reddish-brown). 2. Correct observation for acidic/neutral solutions (remains yellow). 3. Acknowledges the limitation of turmeric in distinguishing between acidic and neutral solutions. 4. Suggests a method to further differentiate (optional but shows comprehensive understanding).

Grading Rubric: 1. Correct definition of neutralization (acid + base \rightarrow salt + water). 2. Mentions the release of heat. 3. Provides a correct example of an acid and a base. 4. Correctly identifies the salt and water as products. 5. Correctly writes the chemical representation/equation (if included).

Grading Rubric: 1. Provides at least two correct examples for acidic substances. 2. Provides at least two correct examples for basic substances. 3. Provides at least two correct examples for neutral substances. (Minor variations in examples are acceptable as long as they are scientifically accurate for Class 7 level).

Grading Rubric: 1. Suggests two distinct and appropriate home-based indicators (e.g., litmus, turmeric, China rose). 2. Correctly describes the colour change for the basic solution with each chosen indicator. 3. Correctly describes the colour change/no change for the acidic solution with each chosen indicator. 4. Explicitly states the warning against tasting.

Grading Rubric: 1. Identifies the cause of indigestion (excess acid in the stomach). 2. Explains what antacids are (contain bases). 3. Clearly connects the function of antacids to the principle of neutralization. 4. Mentions the outcome of neutralization (reduction of acidity, relief from discomfort).

Let's analyze each sample based on the properties of indicators taught in NCERT Class 7 Science:

• Sample P: The observation states that blue litmus paper turned red when dipped into sample P. This is a characteristic property of acidic substances. Acids are known to turn blue litmus paper red.

• Sample Q: The observation states that red litmus paper turned blue when dipped into sample Q. This is a characteristic property of basic (or alkaline) substances. Bases are known to turn red litmus paper blue.

• Sample R: The observations state that sample R had no effect on either blue or red litmus paper, meaning their colors did not change. Furthermore, when turmeric paper was used with sample R, no color change was observed (turmeric paper turns reddish-brown in basic solutions). These combined properties indicate that sample R is a neutral substance. Neutral substances do not change the color of litmus paper, nor do they typically react with turmeric to show a color change.

Therefore, the correct classification for samples P, Q, and R, respectively, is Acidic, Basic, and Neutral. Option A correctly matches this classification, while the other options provide incorrect combinations for the observed indicator changes.