Monday, 20 May 2013

Science Week Fun

We are being Scientists today.

We know that Scientists

We are doing an experiment with

Food Colouring
Dish washing liquid

Before we start our experiment we are going to predict what happens

I think it will make sticky stuff and lots of bugs will stick to it - Bryn

I think it will bubble up and it will have lots and lots of colours mixed up together - Tiaan

I think it will bubble up into a colourful chemical reaction - Riley B

I think it will bubble up and get all over us - Ashley

I think it will turn into potions - Grace

I think it will explode like a volcano - Emma

Now it's time to test it out


we put milk into our plate
then we put drops of colour into the milk
Finally we add little bits of dish washing liquid on a tooth pick

What Happened???

when we were putting the dishwashing liquid into the bowl where the food colouring was it kind of like exploded. - Tiaan

when we put the food colouring  in it was really hard because we couldn't move the plate at all- Nethasa

when we put our squirter in it changed all around the plate - Ashley

Here are some photos

Why does it happen?

Milk is mostly water, and water has a property called surface tension, this is because all the water molecules are strongly attracted to other water molecules, but not to air, so they try to get away from the surface of the drop, making the surface as small as possible, this is why raindrops are approximately spherical - the shape with the least surface for its volume.
This means that the surface of water, or milk, is always trying to shrink.
Surface tension in milk
The surface of the milk is always trying to shrink, due to surface tension.
Food Colouring Floating
The food colouring is less dense than the milk so it floats on the surface.
Something else you may have noticed is that the food colouring seems to float on the surface of the milk, this is because the milk has lots of substances dissolved in it such as Calcium making it more dense than  the food colouring which is almost entirely water.
Washing up liquid is designed to break up the surface tension so water can dissolve fats and grease. This means that where you add the washing up liquid the surface tension is much weaker than everywhere else, so this surface gets hugely stretched by the milk which hasn't met the washing up liquid yet.
After you add the detergent
The washing up liquid breaks the surface tension where it lands, allowing this surface to stretch hugely, causing the rest of the surface to shrink hugely.
Convection in Milk
This shrinkage pushes the food colouring downwards, and it floats up again forming beautiful patterns.
Because the rest of the surface is shrinking it must be getting thicker, this pushes the food colouring downwards, and there is a current below the surface flowing back towards the washing up liquid pulling the food colouring along. It then floats back up to the surface producing beautiful patterns.

Why does washing up liquid reduce surface tension?

A washing up liquid molecule  is made up of a water loving head and a water hating tail, so when you add it to water the molecules arrange themselves over the surface - head inwards. The water is strongly attracted to the heads of these molecules, so is now  stops trying to reduce its surface area, and the surface tension is far weaker.


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