Case Study:

After three days of continuous heavy rain, residents living near a lake on the outskirts of an industrial town noticed something unusual. Hundreds of fish were floating dead on the surface of the water early in the morning. Surprisingly, the lake water still looked almost normal from the outside. There was no major foul smell, and the colour of the water had changed only slightly.

Environmental officers collected water samples from different parts of the lake. Laboratory testing showed that rainwater had washed industrial waste containing iron compounds, chemical dyes, and organic substances into the lake.

If you've ever wondered why certain metals react so specifically with the environment, explore why iron rusts and other real-life applications of chemical reactions to see the science behind the decay.

Scientists found that the dissolved oxygen level in the lake had dropped from 8 mg/L to only 2 mg/L overnight.  This sudden shift is a classic example of how chemical changes impact our world; you can dive deeper into the different types of chemical reactions explained with real-life examples to better identify these patterns. At the same time, oxidation reactions were occurring rapidly in the water.

Some chemicals present in the waste reacted with oxygen dissolved in water. During these chemical reactions, oxygen was continuously consumed. As a result, fish and other aquatic organisms did not get enough oxygen for respiration. Rust-coloured deposits were also found near the edges of the lake, indicating oxidation of iron-containing substances.

The investigation revealed that although the lake appeared calm on the surface, several chemical reactions were taking place underwater. These reactions disturbed the natural balance of the aquatic ecosystem and created life-threatening conditions for fish within a few hours.

CASE-BASED QUESTIONS

 MCQ 

Q1. What was the main reason behind the sudden death of fish in the lake?

A. Increase in rainwater level
B. Excess sunlight entering the lake
C. Decrease in dissolved oxygen due to chemical reactions
D. Increase in mud at the bottom of the lake

Q2. The rust-coloured deposits near the lake edges indicate:

A. Freezing of lake water
B. Oxidation of iron compounds
C. Neutralisation reaction
D. Evaporation of salts

 Assertion–Reason 

Q3. Assertion (A): Fish in polluted lakes may die even when the water appears normal from outside.
Reason (R): Oxidation reactions in polluted water can consume dissolved oxygen required by aquatic organisms.
A. Both A and R are true, and R is the correct explanation of A.
B. Both A and R are true, but R is not the correct explanation of A.
C. A is true, but R is false.
D. A is false, but R is true.

 Application-Based 

Q4. During testing, scientists observed that dissolved oxygen decreased rapidly after industrial waste entered the lake. Explain how oxidation reactions can reduce oxygen availability in water.

Q5. Suppose the same polluted water enters another lake with a large number of aquatic plants. Predict one possible effect during daytime and justify your answer.

 Data/Logic-Based 

Q6. The dissolved oxygen level of lake water changed as follows:

Based on the data, which conclusion is most appropriate?
A. Oxygen level increased due to rainfall
B. Chemical reactions in water consumed dissolved oxygen continuously
C. Fish produced excess oxygen in water
D. Rainwater stopped all chemical reactions

 Application + Reasoning 

Q7. A student suggested adding clean flowing water immediately into the polluted lake. How could this step help reduce fish deaths?

ANSWER KEY WITH EXPLANATION

A1. C. Decrease in dissolved oxygen due to chemical reactions
Explanation: Oxidation reactions consumed dissolved oxygen present in water. Fish require dissolved oxygen for respiration, and low oxygen levels can lead to sudden death.

A2. B. Oxidation of iron compounds
Explanation: Rust formation is a common example of oxidation. Iron-containing substances react with oxygen and moisture to form rust-coloured compounds.

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A3. A. Both A and R are true, and R is the correct explanation of A.
Explanation: The lake looked normal externally, but oxidation reactions inside the water reduced dissolved oxygen levels. This directly affected aquatic life.

A4. During oxidation reactions, substances present in industrial waste react with dissolved oxygen in water. Continuous consumption of oxygen lowers oxygen availability for aquatic organisms, causing stress or death.

A5. Aquatic plants may increase oxygen levels during daytime through photosynthesis. This could partially improve dissolved oxygen availability and reduce harm to fish temporarily.

A6. B. Chemical reactions in water consumed dissolved oxygen continuously
Explanation: The data shows a steady decrease in oxygen concentration after waste entered the lake. This indicates ongoing chemical reactions consuming oxygen.

A7. Adding clean flowing water can increase dissolved oxygen levels and dilute harmful chemicals. This may improve survival conditions for fish and other aquatic organisms.

Ready to test your knowledge further? You can sharpen your skills with our Class 10 Chemistry worksheets. For those looking to simulate exam conditions, try our unsolved practice papers, or review your progress with our fully solved practice papers.

HOTS EXTENSION QUESTIONS

Q1. If industrial waste containing reducing agents instead of oxidising substances entered the lake, how might the oxygen balance of the water change? Explain your reasoning.

Q2. Suggest one long-term scientific solution industries can adopt to prevent oxidation-related oxygen depletion in nearby water bodies.