Match each water treatment chemical or industrial effluent contaminant on the left with it… — JAMB UTME

Match each water treatment chemical or industrial effluent contaminant on the left with its correct chemical action or environmental impact on the right.

Calcium hydroxide, $Ca(OH)_2$ , and sodium carbonate, $Na_2CO_3$ Precipitates $Ca^{2+}$ and $Mg^{2+}$ ions to remove both temporary and permanent water hardness
Non-biodegradable alkylbenzene sulfonate synthetic detergentsForms persistent surface foam that hinders light penetration and retards atmospheric re-aeration
Industrial effluent with high Biochemical Oxygen Demand ( $BOD$ )Stimulates rapid aerobic bacterial decomposition, causing severe depletion of dissolved oxygen
Soluble lead ( $Pb^{2+}$ ) and mercury ( $Hg^{2+}$ ) heavy metal ionsBioaccumulates in aquatic food chains, causing persistent neurological damage in organisms

Answer

The correct pairings connect calcium hydroxide and sodium carbonate to the precipitation of hardness-causing cations; non-biodegradable detergents to persistent foam formation that inhibits aeration; high BOD effluent to microbial oxygen depletion; and heavy metal ions to trophic bioaccumulation and neurological toxicity.

The correct pairings accurately link chemical reagents (

Ca(OH)_2, Na_2CO_3

) to precipitation of hardness-causing metal ions, non-biodegradable synthetic detergents to persistent surface foam formation, high BOD organic waste to microbial depletion of dissolved oxygen, and heavy metal ions (

Pb^{2+}, Hg^{2+}

) to biological biomagnification.

Step-by-Step Solution

Analyze the chemical function of lime and soda ash in water treatment

Calcium hydroxide (

Ca(OH)_2

) converts soluble

Ca(HCO_3)_2

into insoluble

CaCO_3

, and sodium carbonate (

Na_2CO_3

) precipitates

Ca^{2+}

and

Mg^{2+}

ions as insoluble carbonates (

CaCO_3

and

MgCO_3

Chemical precipitation using lime-soda softening removes both temporary and permanent water hardness.

Evaluate the environmental impact of branched alkylbenzene sulfonate detergents

Because soil and aquatic microbes lack enzymes to degrade branched-chain alkylbenzene sulfonates, these surfactants accumulate as stable surface foams.

Surface foam restricts light transmission required for photosynthesis and reduces re-aeration at the water-air boundary.

Relate Biochemical Oxygen Demand (BOD) to organic pollution effects

High BOD signifies extensive organic pollution, fueling exponential growth of decomposer bacteria that rapidly absorb dissolved oxygen.

Aerobic respiration by decomposers lowers the dissolved oxygen concentration, creating hypoxic aquatic conditions.

Assess the biological toxicity of heavy metal effluents

Heavy metal ions such as

Pb^{2+}

and

Hg^{2+}

are non-biodegradable toxins that accumulate in fatty tissues and biomagnify up food chains.

Inhibition of key metabolic enzymes by heavy metals causes chronic neurological and physiological damage.

Key Concept

Chemical Methods of Water Softening, Effluent Pollution Mechanisms, and Ecological Impacts

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