Revision: Practical Work Chemistry (English Medium) ICSE Class 9 CISCE

Oil spills are the accidental discharges of petroleum into oceans or estuaries. The sources of spills are overturned oil tankers, offshore oil mining, and oil refineries.

Water pollution means any change in the water quality which makes it unsuitable for use by humans and by other living organisms.

The pOH of a solution can be defined as the negative logarithm to the base 10, of the molar concentration of OH⁻ ions in solution.

pOH = -log₁₀[OH^-]

The pH of a solution is defined as the negative logarithm to the base 10, of the concentration of H⁺ ions in solution in mol dm^–3.

pH is expressed mathematically as

pH = -log₁₀ [H⁺] or pH = -log₁₀ [H₃O⁺]

pH scale is a scale for measuring the hydrogen ion concentration in a solution.

The amount of heat that is needed to raise the temperature of a unit mass of a substance by 1°C is called specific heat capacity.

• Blue/green salts may indicate Cu²⁺, Fe²⁺, or Fe³⁺ ions.
• White salts can suggest Pb²⁺, Zn²⁺, Ca²⁺, Na⁺, K⁺, or NH₄⁺ ions.
• Ammonia smell suggests NH₄⁺, H₂S smell indicates sulphides, and SO₂ smell indicates sulphites.
• Amorphous salts may indicate carbonates (CO₃²⁻).
• Hygroscopic/deliquescent salts point to Cl⁻ or NO₃⁻ ions.

• The Water (Prevention and Control of Pollution) Act, 1974, was passed to safeguard water resources. Main sources — domestic sewage, industrial effluents and agricultural run-off.
• BOD (Biochemical Oxygen Demand) is the dissolved oxygen required by microorganisms to decompose organic matter in water. High BOD = high pollution = low oxygen = death of aquatic organisms.
• Algal bloom — excessive growth of planktonic algae due to excess nitrogen and phosphorus. Releases toxins and causes fish mortality. Water hyacinth (Eichhornia crassipes) — "Terror of Bengal" — is an invasive plant that chokes water bodies.
• Eutrophication — natural ageing of a lake by nutrient enrichment. When accelerated by human activities, it is called Cultural/Accelerated Eutrophication, depleting oxygen and killing aquatic life.
• Biomagnification — increase in concentration of toxic pollutants (DDT, mercury) at successive trophic levels. Non-degradable, accumulate in tissues and pass to the next trophic level.
• Thermal pollution — caused by thermal and nuclear power plants releasing hot coolant water, raising water temperature and killing aquatic flora and fauna.

pH = negative logarithm of H₃O⁺ ion concentration (mol/L).

• The pH scale (0–14) measures the concentration of H⁺ ions in a solution; values < 7 indicate acids, > 7 indicate bases, and 7 is neutral.
• A universal indicator shows different colours at different pH levels, helping to determine the strength of an acid or base.
• Strong acids/bases produce more H⁺ or OH⁻ ions in solution, while weak acids/bases produce fewer ions at the same concentration.

• Proper maintenance of water bodies and avoiding the disposal of waste into them helps reduce sewage pollution.
• Use of pesticides and chemical fertilisers should be minimised to prevent water contamination.
• Sewage should be properly treated before being released into rivers and other water bodies.
• Ecological sanitation (Ecosan) uses dry composting toilets to recycle human waste into organic manure, reducing the need for fertilisers.
• Ecosan toilets save water, prevent groundwater contamination, and are useful in water-scarce areas.
• Recycling of sewage water using reverse osmosis (RO) helps solve water scarcity and waste disposal problems.
• Rainwater harvesting and sewage recycling in cities help conserve water and reduce sewage generation.