What characteristic does not vary in water used for domestic purposes?

The characteristic that does not vary in water used for domestic purposes is the amount of minerals in the water. This is because, while different sources of water can indeed have different mineral compositions, once the water has been treated and meets regulatory standards for domestic use, it tends to maintain a relatively consistent range of mineral content suitable for consumers.

For example, municipal water supplies are treated to ensure they have specific levels of minerals, which helps maintain safety and palatability. This standardization is crucial as it helps to prevent fluctuations that might otherwise occur with varying input sources like groundwater or surface water, which can contain different levels of hardness or specific mineral content.

In contrast, the temperature of water can vary significantly based on environmental conditions or heating systems, water sources can differ widely (ranging from well water to rivers), and the pH can fluctuate due to various factors such as contamination or the presence of certain minerals. Thus, the amount of minerals remains the most consistent characteristic in treated domestic water.