Understanding Water Colour and Impurities

Water quality is affected by various substances present in it. These substances can be broadly classified based on their size and how they behave in water: suspended impurities, dissolved impurities, and colloidal impurities.

The question asks what generally causes the colour in water. Let's examine the options:

Examining Impurities and Water Colour

• Suspended Impurities: These are particles that are large enough to be seen by the naked eye and can be filtered out. Examples include sand, silt, and clay. While they make water look cloudy or turbid, they typically don't impart a uniform colour to the water body itself, although the particles themselves may have colour. Turbidity is the cloudiness caused by suspended solids.
• Dissolved Impurities: These are substances that are completely dissolved in water at a molecular or ionic level. They are not visible and cannot be removed by simple filtration. Many dissolved substances, such as certain minerals (like iron or manganese) or organic compounds (like tannins from decaying vegetation), absorb or scatter light in specific ways, giving the water a distinct and uniform colour. For example, dissolved iron can make water appear reddish-brown, while dissolved organic matter can give it a yellow or brown tint.
• Colloidal Impurities: These are particles smaller than suspended solids but larger than dissolved molecules. They remain dispersed in water without settling due to their small size and often a surface charge. Colloidal particles can contribute to turbidity and, in some cases, colour, but dissolved substances are more often the primary cause of true, uniform colouration in water that is not turbid.

Conclusion on Water Colour Cause

Based on the nature of these impurities, dissolved substances are the most common cause of the actual colour perceived in water. While suspended and colloidal particles affect turbidity (cloudiness), it is the dissolved chemicals and organic matter that absorb or reflect light to create colour.

Therefore, the colour in water is generally due to dissolved impurities.

Revision Table: Water Quality Parameters

Additional Information: Specific Causes of Water Colour

The specific colour observed in water can often provide clues about the type of dissolved impurities present:

• Yellow to Brown: Often caused by dissolved organic matter, such as humic and fulvic acids, which are decomposition products of vegetation (tannins). This is common in water from peaty or swampy areas.
• Reddish-Brown: Commonly due to dissolved iron, which oxidizes upon contact with air to form insoluble iron compounds.
• Black/Dark Brown: Can be caused by dissolved manganese or high concentrations of organic matter.
• Green: Usually indicates the presence of algae (which are living organisms, not typically considered dissolved or suspended in the conventional sense causing uniform colour, but their pigments give the water a green tint). However, dissolved substances like copper salts can also impart a bluish-green colour.

Understanding the cause of colour is important in water treatment processes, as different types of impurities require different removal methods.