All questions of July Week 3 for Class 10 Exam

Understanding Lime Water in Fermentation
In the fermentation experiment, the milky appearance of lime water is a significant observation. This phenomenon is primarily due to the release of carbon dioxide during the fermentation process. Here’s a detailed explanation:
What is Lime Water?
- Lime water is a dilute solution of calcium hydroxide (Ca(OH)2).
- It is clear and colorless when undisturbed.
What Happens During Fermentation?
- Fermentation is a metabolic process where sugars are converted into alcohol or acids by microorganisms like yeast.
- During this process, carbon dioxide (CO2) is produced as a byproduct.
Formation of Calcium Carbonate
- When carbon dioxide is bubbled through lime water, it reacts with calcium hydroxide, forming calcium carbonate (CaCO3).
- The chemical reaction can be summarized as follows:
- Ca(OH)2 + CO2 → CaCO3 (precipitate) + H2O
Indication of Carbon Dioxide Release
- The formation of calcium carbonate is what causes the milky appearance of the lime water.
- The precipitate makes the solution appear cloudy or milky, indicating that carbon dioxide is present in the solution.
Conclusion: Why Option 'C' is Correct
- The milky appearance signifies the release of carbon dioxide during fermentation, confirming option 'C' as the correct answer.
- Thus, the observation serves as a practical indicator of the fermentation process occurring, validating the metabolic activity of the microorganisms involved.

Understanding Rainwater Purity
Rainwater is often considered one of the purest forms of natural water in certain regions due to several factors related to its formation and collection process.
Natural Purification Process
- Rainwater originates from the evaporation of water from the Earth's surface.
- As water vapor rises, it cools and condenses into droplets, forming clouds.
- During this process, most impurities and contaminants such as minerals, bacteria, and pollutants are left behind.
Minimal Human Interaction
- Rainwater falls directly from the atmosphere, meaning it is generally untouched by human activities.
- In remote or less industrialized areas, rainwater is less likely to come into contact with pollutants from urban runoff, industrial waste, or agricultural chemicals.
Collection Environment
- In regions with minimal air pollution, rainwater can be collected without significant contamination.
- The collection systems, often designed to capture rain directly from roofs or open surfaces, can be kept clean, maintaining the water’s purity.
Freshness and Nutrient Content
- Fresh rainwater can also contain beneficial nutrients like nitrogen, which can be advantageous for plants.
- It provides a natural resource that is often softer and more palatable compared to groundwater, which may have higher mineral content.
In summary, rainwater is considered among the purest forms of natural water because it is largely free from human-induced pollutants and contaminants, especially in regions where environmental conditions are favorable.

Understanding the Role of Yeast in Fermentation
Fermentation is a crucial biochemical process in which yeast converts sugars into alcohol and carbon dioxide. When yeast is added to fruit juice or a sugar solution, it plays several important roles.
Carbon Dioxide Production
- The primary reason for adding yeast is to facilitate the production of carbon dioxide (CO2).
- During fermentation, yeast consumes sugars, resulting in the byproduct of CO2, which creates bubbles and a fizzy texture in beverages like sparkling wine or beer.
Alcohol Formation
- Alongside CO2, yeast also produces alcohol (ethanol) as a byproduct.
- This transformation is fundamental in producing alcoholic beverages, where the desired effect is the creation of alcohol for consumption.
Flavor Development
- The fermentation process contributes to the development of unique flavors in the final product.
- Yeast can create a variety of compounds that enhance the taste profile of the beverage, but this is secondary to CO2 production.
Why Not the Other Options?
- a) Introducing a sweet flavor is incorrect because yeast converts sugars into alcohol, which generally reduces sweetness.
- b) While yeast does speed up the fermentation process, its primary role is to facilitate carbon dioxide and alcohol production.
- d) Yeast does not change the color of the solution; that is usually determined by the fruit juice or sugar used.
In summary, adding yeast to fruit juice or sugar solutions primarily serves to produce carbon dioxide, which is essential for creating the desired characteristics in fermented beverages.