Mixtures and their Separation
Mixtures, concentration and separation
Tell solutions, suspensions and colloids apart, work out a concentration as a percentage, and choose the right method for pulling a given mixture apart.
Stir salt into water and the salt vanishes: the mixture is the same throughout, so it is homogeneous, and a homogeneous mixture is what we call a solution. Stir sand into water and the grains stay in plain view, then sink — that mixture is heterogeneous. Milk in water looks like the first and behaves like the second, and sorting out which is which is what a laser beam is for.
Three sizes of particle
The three kinds of mixture differ by how big the scattered particles are. Under 1 nanometre and you have a solution: clear, and a light beam passes through unseen. Between 1 and 1000 nanometres and you have a colloid: it scatters light, which is the Tyndall effect, but nothing settles. Over 1000 nanometres and you have a suspension: the particles are visible, they scatter light, and given time they sink.
Have a play
Three beakers, and what the laser beam does in each of them.
Tap any part of the picture.
Worked example
Three beakers are set up: salt in water, chalk powder in water, milk in water. A laser is shone through each. The beam's path is invisible in the first and clearly visible in the other two. Left standing, the second one settles and the third does not. What are the three?
Start with the beaker where the beam vanished: its particles are too small to scatter light, so they are under a nanometre. That is the salt solution.
Scattering needs something of a size worth scattering off. This is the test that separates a true solution from the other two.
Try it together
Now put a number on a concentration.
A solution is made by dissolving 15 g of salt in 85 g of water. Answer with a bare number each time.
1.First, what is the mass of the whole solution, in g?
Have a go
Have a go on your own. 6 g of sugar is dissolved in 194 g of water. What is the mass by mass percentage of the solution? Answer with a number.
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.