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.

Lemon juice is acidic. To neutralize an acid, a base is needed. Baking soda solution is basic and will neutralize the lemon juice.

Acidity in the stomach is caused by excess acid. Milk of magnesia is a mild base (magnesium hydroxide) that can neutralize stomach acid and relieve acidity.

Turmeric paper turns reddish-brown in the presence of a basic substance. It remains yellow with acidic or neutral substances.

Olfactory indicators change their smell in acidic or basic media. Since Manisha is blindfolded, she cannot see color changes, making an olfactory indicator suitable for her to distinguish between acidic and basic solutions by smell.

Bases are substances that produce hydroxide ions ($OH^-$) when dissolved in water. Therefore, the common ion present in all bases is the hydroxide ion.

Yes, it is a good idea to take a soap bath if an ant bites you. Ant stings contain formic acid, which causes irritation. Soap is basic in nature and can neutralize the formic acid, thereby providing relief from the sting.

Here are the names of acids and bases present in common food items: 1. Vinegar: Acetic Acid 2. Curd: Lactic Acid 3. Soap: Sodium Hydroxide/Potassium Hydroxide (Base) 4. Window cleaner: Ammonium Hydroxide (Base)

Neutralization reaction is a chemical reaction in which an acid and a base react quantitatively with each other to form a salt and water. For example, when hydrochloric acid ($HCl$) reacts with sodium hydroxide ($NaOH$), it forms sodium chloride ($NaCl$) and water ($H_2O$). The general equation is: Acid + Base $\rightarrow$ Salt + Water. This reaction essentially neutralizes the acidic and basic properties of the reactants.

Red rose extract acts as a natural indicator. It turns red in acidic solutions and green in basic solutions. Grape juice is acidic, so when mixed with red rose extract, the mixture will be red. Baking soda is basic. When baking soda is added to this red mixture, it will neutralize the acid from the grape juice and make the solution basic. Consequently, the mixture will change its color from red to green, indicating the presence of a base.

To draw a contrast matrix (table) for acid and base based on the given parameters, you would set up a table with 'Parameter' as the first column, 'Acid' as the second column, and 'Base' as the third column. Then, fill in the contrasting characteristics for each parameter:

1. Taste:

   • Acid: Generally tastes sour (e.g., lemon, vinegar). Important Note: Never taste unknown chemicals.
   • Base: Generally tastes bitter and feels soapy to touch (e.g., baking soda solution). Important Note: Never taste unknown chemicals.

2. Litmus test:

   • Acid: Turns blue litmus paper red.
   • Base: Turns red litmus paper blue.

3. Nature:

   • Acid: Are acidic substances. They can be corrosive. In aqueous solutions, they typically produce hydrogen ions (H+).
   • Base: Are basic or alkaline substances. They can be caustic. In aqueous solutions, they typically produce hydroxide ions (OH-).

4. pH example (Any 3): The pH scale measures the acidity or alkalinity of a solution. A pH of 7 is neutral, less than 7 is acidic, and greater than 7 is basic.

   • Acid: Have a pH value less than 7.
     • Examples: Lemon juice (pH ~2-3), Vinegar (pH ~2.5-3.5), Stomach acid (pH ~1.5-3.5).
   • Base: Have a pH value greater than 7.
     • Examples: Baking soda solution (pH ~8-9), Soap solution (pH ~9-10), Household bleach (pH ~12-13).

<u>Flow chart for making natural indicators:</u> <b>1. Turmeric Indicator:</b> 1. Take a small amount of turmeric powder. 2. Add a little water to make a paste. 3. Spread the paste on a filter paper or blotting paper. 4. Let it dry. The yellow paper is now a turmeric indicator. <b>2. Red Rose (China Rose) Indicator:</b> 1. Collect petals of a red rose (China rose). 2. Place the petals in a beaker. 3. Add some warm water to the beaker. 4. Let it stand for some time until the water becomes colored. 5. Filter the colored water to get the China rose indicator solution.

China rose indicator turns green in basic solutions. Sodium hydroxide is a strong base. Therefore, the beaker that contains sodium hydroxide will show a green color when tested with China rose water. Based on the image, <b>Beaker C</b> has the green solution, indicating it contains sodium hydroxide. Beaker A (red) would be acidic, and Beaker B (pink/magenta) would be neutral or very weakly acidic/basic.

Turmeric is a natural indicator that changes color in acidic and basic solutions. Soap solution, vinegar, and baking soda are substances, not indicators.

Basic substances turn red litmus paper blue. Soap solution is basic, while lemon juice, vinegar, and tamarind juice are acidic.

Lemon juice is acidic. Baking soda solution, lime water, and detergent solution are all basic substances.

A salt is typically formed in a neutralization reaction between an acid and a base, producing salt and water.

Turmeric is a natural indicator that turns reddish-brown in basic solutions. Both the assertion and the reason are true, and the reason correctly explains why turmeric changes color in a base.

The provided option accurately lists multiple key differences between acids and bases, including their taste, effect on litmus paper, pH range, and physical properties or reactivity.

The option correctly defines indicators as substances that change color in acidic or basic solutions and provides two natural examples, turmeric and litmus, along with their characteristic color changes.

The option comprehensively answers all parts of the question by defining neutralization, identifying salt and water as products, and providing a relevant everyday application.