# Dissolving and Filtering

In this worksheet, students will be helped to practise their understanding of what dissolving is about and how that helps certain substances to be separated from each other.

Key stage:  KS 2

Curriculum topic:   Properties and Changes of Materials

Curriculum subtopic:   Dissolving and Solutions

Difficulty level:

### QUESTION 1 of 10

Separating substances from each other can be a difficult problem. However there are certain 'rules' that actually help you to make sense of it, like if you've mixed two things in water, one which dissolves and the other doesn't, then you can always get them apart by filtering (like a very fine sieve). Similarly, if you want to get the dissolved substance back out of the water, you simply evaporate the water away.

Let's practise some of these ideas and see how you get on......

Evie and Charlotte had a mixture of pebbles, sand and salt - what would be their best choice for separating the pebbles from the sand and salt?

sieve

filter

cup of water

Next they stirred the sand/salt mixture up in some water.

What did the sand do?

dissolve

sink to the bottom

mix with the water

When Evie and Charlotte stirred the salt and sand into the water, what happened to the salt?

it sank to the bottom

it disappeared

it dissolved

Now Evie and Charlotte wanted to separate the salty water from the sand - they're all mixed up together.

What is the proper scientific term for the salty water?

sea water

salt mixture

salt solution

To try to separate the sand from the salty water Evie and Charlotte tried using a sieve.

What do you think happened when they poured the mixture into the sieve?

the sand got stuck in the sieve

the salt got stuck in the sieve

both sand and salt passed through the holes

Next they tried a filter. This uses a special filter paper (like in a coffee machine) with tiny holes in.

Evie and Charlotte put a piece of filter paper in a funnel and poured the salty water/sand mixture into it.

What do you think they saw left behind in the filter paper?

sand

salt

nothing

Evie said that she thought that she knew how to get the salt out of the water - she would try filtering it on its own.

Charlotte said that she didn't think that would work and she'd prove it: she took a fizzy blackcurrant tablet and dropped it into a glass of water - a pink liquid resulted.  Charlotte poured the liquid into the filter.

What do you think dripped out?

colourless liquid

pink liquid

salty water

Now Evie and Charlotte understood that when things dissolve in water they pass through the holes in the filter paper.

Charlotte said that if they wanted to get the salt back from the salty water they should heat it gently. Here is what they did:

Soon they saw a gas rising above the dish - what colour do you think the gas was?

white

pink

colourless

In the dish Evie and Charlotte saw a ring of white crystals appearing.

What do you think they were made of?

salt

sand

sugar

"There you are!" said Charlotte, "That proves that you can only get the salt back from the water by heating it."

Why do you think that the salt was left behind?

it's a solid and cannot evaporate

the salt is heavier than the water and stays in the dish

mixtures can always be separated by heating

• Question 1

Evie and Charlotte had a mixture of pebbles, sand and salt - what would be their best choice for separating the pebbles from the sand and salt?

sieve
EDDIE SAYS
You need something with fairly big holes to catch the pebbles and let the sand/salt mixture through - a sieve's best.
• Question 2

Next they stirred the sand/salt mixture up in some water.

What did the sand do?

sink to the bottom
EDDIE SAYS
To be honest most of the sand is going to sink to the bottom fairly quickly but it's true that a few of the smaller, finer grains will float around in the water, sinking slowly. Within an hour the water should all be clear with the sand on the bottom.
• Question 3

When Evie and Charlotte stirred the salt and sand into the water, what happened to the salt?

it dissolved
EDDIE SAYS
Oo, tricky! The salt dissolved, meaning that the grains broke up into invisible tiny particles mixed in with the water, so it seems to disappear (but it is still there!).
• Question 4

Now Evie and Charlotte wanted to separate the salty water from the sand - they're all mixed up together.

What is the proper scientific term for the salty water?

salt solution
EDDIE SAYS
When something dissolves in water it actually makes a solution; so when you ask for a cup of tea with one sugar you actually want a weak tea and sugar solution (hot)!
• Question 5

To try to separate the sand from the salty water Evie and Charlotte tried using a sieve.

What do you think happened when they poured the mixture into the sieve?

both sand and salt passed through the holes
EDDIE SAYS
If you remember, back at the beginning of the investigation they used a sieve and both the dry sand and salt passed through it. Now it's all wet it's going to do much the same - a little sand (lumpy) might get caught, but most of it will get through the holes with the salty water.
• Question 6

Next they tried a filter. This uses a special filter paper (like in a coffee machine) with tiny holes in.

Evie and Charlotte put a piece of filter paper in a funnel and poured the salty water/sand mixture into it.

What do you think they saw left behind in the filter paper?

sand
EDDIE SAYS
The tiny holes in the filter paper will be able to trap the grains of sand, and stop them getting through, so they will have seen the sand. The salty water will have passed through the holes with no difficulty.
• Question 7

Evie said that she thought that she knew how to get the salt out of the water - she would try filtering it on its own.

Charlotte said that she didn't think that would work and she'd prove it: she took a fizzy blackcurrant tablet and dropped it into a glass of water - a pink liquid resulted.  Charlotte poured the liquid into the filter.

What do you think dripped out?

pink liquid
EDDIE SAYS
Since the pink colour is due to DISSOLVED SOLIDS they are so tiny that they will pass through the holes in the filter paper with the water.
• Question 8

Now Evie and Charlotte understood that when things dissolve in water they pass through the holes in the filter paper.

Charlotte said that if they wanted to get the salt back from the salty water they should heat it gently. Here is what they did:

Soon they saw a gas rising above the dish - what colour do you think the gas was?

colourless
EDDIE SAYS
Salty water - salt solution - is a mixture of salt and water, so when it's heated the water boils off as steam which is colourless.
• Question 9

In the dish Evie and Charlotte saw a ring of white crystals appearing.

What do you think they were made of?

salt
EDDIE SAYS
Salt solution: water boils away leaving .... yup, salt!
• Question 10

"There you are!" said Charlotte, "That proves that you can only get the salt back from the water by heating it."

Why do you think that the salt was left behind?

it's a solid and cannot evaporate
EDDIE SAYS
A solution is a solid - in tiny tiny particles - mixed with a liquid (like water). When the water is boiled away the solid has to be left behind. It's like fish in a pond in Africa - the rainy season has passed, the sun is getting hotter and hotter. The water level is getting lower as it evaporates away and slowly but surely the fish have less and less water to swim in - they can't evaporate away - they're like solids!
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