Why hot water is preferred to cold water for washing clothes?
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Faster-moving molecules is going to be the reason why hot water is better at washing things than cold water; because faster-moving molecules defines hot water.
Is it warm water 1.1 times better than cold water, as per Jack's answer? Well here is my hypothesis about why, when I'm doing the dishes, it's easier to use hot water:
It's all about melting points. If I'm melting the fat that's stuck my dishes, the dishes'll be easier to clean.
To look at some numbers, hot water comes out of my tap somewhere around 50°C (Google tells me that's 122°F).
Meanwhile, butter (dairy fat) melts between 21 and 36°C[1]. Hydrogenated coconut oil melts between 36 and 40°C[2]. Looks like my hot water will be sufficient to melt food-based fats. It's tough to point to exact melting points because there are various molecular chain lengths and geometries, even if a fat isn't being completely melted under the hot water, part of it is. Liquid oils will become less viscous (less 'thick') as temperatures increase.
Once you are significantly decreasing the viscosity of fats and oils, washing gets much easier; more than a 10% improvement if we calculated a 'cleaning ease' metric.
Edit: It's a good idea to keep in mind exactly what you are cleaning. Hot water will cook proteins and, depending on your dishes, could make it harder to clean well. You're better off cleaning a cup that you drank milk out of with cold water for this reason (think eggs cooking).
Is it warm water 1.1 times better than cold water, as per Jack's answer? Well here is my hypothesis about why, when I'm doing the dishes, it's easier to use hot water:
It's all about melting points. If I'm melting the fat that's stuck my dishes, the dishes'll be easier to clean.
To look at some numbers, hot water comes out of my tap somewhere around 50°C (Google tells me that's 122°F).
Meanwhile, butter (dairy fat) melts between 21 and 36°C[1]. Hydrogenated coconut oil melts between 36 and 40°C[2]. Looks like my hot water will be sufficient to melt food-based fats. It's tough to point to exact melting points because there are various molecular chain lengths and geometries, even if a fat isn't being completely melted under the hot water, part of it is. Liquid oils will become less viscous (less 'thick') as temperatures increase.
Once you are significantly decreasing the viscosity of fats and oils, washing gets much easier; more than a 10% improvement if we calculated a 'cleaning ease' metric.
Edit: It's a good idea to keep in mind exactly what you are cleaning. Hot water will cook proteins and, depending on your dishes, could make it harder to clean well. You're better off cleaning a cup that you drank milk out of with cold water for this reason (think eggs cooking).
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