快猫短视频

Is hard or soft water better at removing dirt and bacteria?

鈥� The difference hard water makes to the effectiveness of washing depends on the type of hardness, dirt and detergent. As a rule of thumb, hard water interferes with cleaning because it contains dissolved salts that cause some common kinds of detergent to precipitate out, more or less gluing down grit, grease and other dirt, especially in corners, cracks and on rough surfaces.

If the dirt is full of harmful microbes, then they become hard to remove or to expose to disinfectants and sterilising ultraviolet. This is especially true for spores.

So such encrusted dirt not only looks grubby and is deeply unattractive, it is also likely to be a health risk. Sparkling clean surfaces may still be far from sterile, but they are rather more encouraging than grimy ones.

鈥淥ne panellist advocates urinating directly into your compost heap whenever you work in the garden鈥�

Conversely, on surfaces that don鈥檛 support encrustation from the hard water and detergent in question, the precipitation of solids might be welcome. If they don鈥檛 stick, they can be easily rinsed away, taking the microbes with them. This is preferable to leaving millions of live bacteria and spores where apparently clean residual water films have dried.

Jon Richfield, Somerset West, South Africa

鈥� Hard water contains more dissolved calcium and magnesium salts than soft water does. These salts leave a visible residue on freshly cleaned glassware or shiny black cars, for example, when drops of the final rinse water evaporate after cleaning. They can also give a dull, coarser-textured finish to hair and textiles 鈥� so a final rinse with soft water can improve the cleaning process.

Cleaning is often carried out using detergents, which generally work better in places with soft water. As a result, manufacturers recommend using extra detergent when washing clothes in hard-water areas.

The reason for the poorer performance is that dissolved salts actually increase the surface tension of water. Detergents do the opposite, reducing surface tension and improving the mobilisation of dirt, oils and fats.

Traditional soap, made from alkali-treated fat, is also less effective in hard water. Its soluble, fatty-acid detergent salts are precipitated by the hard-water salts. This is why, for instance, a 鈥渟cum鈥� forms round the edge of the bath.

As for bacteria, they often excrete viscous, water-soluble, extracellular polymers to help them adhere to surfaces. These polymers are the basis of so-called biofilms. Soluble inorganic salts generally thicken such polymers. Therefore hard water tends to consolidate biofilms, rather than remove them, and soft water is better than hard at removing bacteria in a typical biofilm.

Mark Wareing, Macclesfield, Cheshire, UK

We pay 拢25 for every answer published in 快猫短视频. To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.

Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.

快猫短视频 retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.

You can also submit answers by post to: The Last Word, 快猫短视频, 25 Bedford Street, London WC2E 9ES.

Terms and conditions apply.