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Learn/ 12th Class Chemistry/ Unit 33 /Short Questions

Unit 33: Water — Short Questions

12th Class Chemistry · Unit 33: Water

Q2.a.Differentiate between Point Source and Non-Point Source pollution in terms of the "concentration" of pollutants at the site of entry.

Point source pollution enters a water body from a single, discrete, identifiable location (e.g. a discharge pipe), so the pollutant concentration is highest right at and downstream of that site, producing a concentrated "pollution plume" and localized acute toxicity. Non-point source pollution is diffused — pollutants are picked up by rainfall/snowmelt over a wide area and enter the water body at many points, so at the site of entry the pollutant concentration is much lower, though spread across a larger area.

Q2.b.Using an example from the text, explain how an underground storage tank can become a point source of pollution.

A storage tank at a chemical plant can leak its contents, which seep into the soil and subsequently into the groundwater. Although the tank is underground and the leak is slow, the contamination still originates from one single, identifiable location, so it is classified as a point source of pollution rather than a non-point source.

Q2.c.Identify two types of chemical pollutants commonly found in urban storm water runoff.

Leaked engine oils (hydrocarbons) and road salts (chemicals washed off roads/car parks by stormwater); zinc from tyre wear is another example.

Q2.d.Differentiate between the "solubility" of pesticides like DDT and inorganic nutrients like nitrates in the context of the water cycle.

Nitrates are ionic, so water's polarity readily dissolves them, letting them move freely and widely through the water cycle (causing eutrophication). Pesticides like DDT, however, are non-polar and essentially insoluble in water; instead they are highly soluble in the fatty tissues of organisms, so they do not disperse through water itself but accumulate (bioaccumulate) in the fatty tissues of fish and other organisms.

Q2.e.Explain why microplastics are considered a "growing environmental crisis" differently than organic solid waste.

Organic solid waste eventually biodegrades. Plastics, however, do not biodegrade; they only break down into smaller and smaller microplastic particles that persist in the environment for centuries. These microplastics are ingested by marine animals (causing choking or internal injury) and have entered the food chain, raising concerns for human health, which organic waste decomposition does not cause in the same way.

Q2.f.How does an oil spill physically disrupt the chemical balance of a marine ecosystem?

Spilled oil spreads rapidly across the water surface, forming a layer that blocks sunlight and reduces oxygen exchange between the air and water. This lowers dissolved oxygen and harms marine life: mammals ingest toxic oil while grooming, fish suffer long-term health effects from hydrocarbon exposure, and coastal habitats such as mangroves and coral reefs are damaged.

Q2.g.Why are synthetic organic pollutants, like PCBs and pesticides, considered "persistent" in the environment?

Compounds like DDT and PCBs are persistent organic pollutants (POPs) that are chemically stable and non-biodegradable. They resist natural breakdown, so they remain in the environment and in fatty tissues for long periods, accumulating (bioaccumulation) and becoming more concentrated up the food chain (biomagnification).

Q2.h.How do the decomposition of organic matter in polluted water contributes to global warming?

Sewage, agricultural runoff, or industrial waste entering water boosts the growth of algae and other organisms. When these die and decompose under oxygen-poor (anaerobic) conditions, methane — a potent greenhouse gas — is released, which contributes to climate change and worsens global warming.

Q2.i.Name four methods used for water conservation.

1. Water-saving irrigation technologies such as drip and sprinkler irrigation. 2. Reusing and recycling greywater (laundry, dishwashing, bathing water) for toilet flushing, landscaping, or industrial cooling. 3. Rainwater harvesting systems in urban and rural areas. 4. Regular repair and maintenance of water supply infrastructure to minimize leaks (constructing small dams/recharge wells and community awareness campaigns are also valid answers).

Q2.j.What is the role of "industrial discharge" in water pollution?

Industrial activities are a major source of water pollution: factories (oil refineries, chemical plants, automobile industry, fertilizer plants, tanneries, etc.) often discharge harmful chemicals, heavy metals, and toxic waste directly into rivers, lakes, and oceans, usually as point-source pollution.

Q2.k.a.Both reverse osmosis (RO) and ion exchange methods are used to obtain high-purity water. Differentiate between the processes involved in both these methods.

Reverse osmosis is a physical separation process: water is forced under pressure through a semi-permeable membrane that filters out dissolved salts, bacteria, and other impurities. Ion exchange is a chemical substitution process: as water passes through an ion-exchange column, undesirable ions (Ca²⁺, Mg²⁺) are swapped for Na⁺ or H⁺ ions held on the resin.

Q2.k.b.Both reverse osmosis (RO) and ion exchange methods are used to obtain high-purity water. What impurities do these methods target?

RO targets a broad range of impurities — dissolved salts, bacteria, and other contaminants — producing very high-purity water. Ion exchange specifically targets hardness-causing ions, Ca²⁺ and Mg²⁺, replacing them with Na⁺ or H⁺ ions to "soften" the water.