24th March – Soaps and Water Tension (subtitled)

24th March – Soaps and Water Tension (subtitled)


Okay so today I am going to be looking at how soaps affect the surface tension of water so i’m going to be using same nice hand soap which is in short supply at the moment and some peppercorns and i’ve got my bowl of water here and i’ve got a pestle and mortar. So I put some peppercorns in here so i’m going to grind them up so i’ve got nice granules I really like this experiment because it works so well so i’ll put them into the water so one of the observations we’re going to note is that the pepper sitting on top of the water there is some that have floated to the bottom as they are quite heavy but the majority of the pepper has floated on top Ok so what I am going to do is grab my soap lovely love hearts smell some on my finger and then what i’m going to do i’m just going to pop my finger in and as you can see the pepper moves away so soap is still on my hand… still moving and it goes into globules so… why does it do this? why does the pepper float? so it is hydrophobic hydrophobic means it doesn’t like water, so why is it not on the bottom the water is held together by hydrogen bonding which is an intermolecular force and as you can see i’ve got the water molecules here and the dotted lines show the intermolecular forces the hydrogen bonds between the water molecules, so this gives more surface tension on top of the water so that is why they float on top of the water but why does the pepper shoot to the side? we are using a soap so soaps are made from alkaline hydrolysis of fats and oils. so hydrolysis of fats and oils looks like this so we’ve got a fat or an oil and add sodium hydroxide which is an alkali and that is why it is alkaline hydrolysis and we get soap and glycerol this is called saponification and that is the process of making a soap OK so why is this important? so for the example of the soap we’re using we’re using carex love hearts and as you can see from the ingredients it says that it is aqua, then sodium laureth sulfate OK so sodium laureth sulfate so sodium laureth sulfate is actually called a detergent or surfactant and it reduces the surface tension in the water so as you can see here, i’ve got an example of what sodium laureth sulfate looks like and as you can see we have a negatively charged head here and it is hydrophilic as it is negatively charged and uncharged covalent tail which is hydrophobic so the hydrophobic tail when we add this soap to the water is repelled by the water and therefore it shoots the pepper to the side along with it and that is also why we get these globules in the water so it i turn it round like this you can see that is has kind of clumped together i’ve still got some soap on my hand yep, so it’s still moving so that is the action of soap so this is the same, this kind of shows what grease will do as well so grease as well will sit on top of the water and then once you add that soap the grease shoots away from the soap.

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