Glossary

Bar chart a graph using bars to show separate pieces of data

Condensation the change from gas or vapour to liquid- Water vapour condenses on a cold surface

Dissolve when a solid spreads evenly through a liquid and the solution becomes clear. The solid does not separate out

Evaporate when a liquid becomes a vapour

Emulsion A mixture of 2 insoluble liquids e.g. water and oil

Evidence the facts that support a discovery, an idea or argument

Filter separating a solid and a liquid using a material like paper

Foam A mixture of liquid and an un-dissolved gas

Gas a material with weight but no shape which moves freely through any space available

Line graph a graph made by joining related pieces of data together to make a straight line or curve

Liquid a material that can flow- It takes the shape of the container you put it in and forms a level surface

Solid a material that keeps its shape- You can pour some solids, but they do not pour to form a level surface

Solution a solid spread evenly through a liquid- you can’t filter out a dissolved solid

Suspension A mixture of liquid and an un-dissolved solid which is floating in the liquid

Vapour like gas

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Filtering and sieving

Some solids don’t dissolve in water. These insoluble solids can be separated. Filtering is similar to sieving

The photograph shows some river water. It is not pure because it has other things in it. It is a mixture of water and other things. A pure substance ha nothing else

added to it. For example, pure water only contains water.

When you put some solids in water, they sink to the bottom or float on the top. They do not dissolve. These are insoluble solids.

If you want to get the clean water back, you can separate solid from the liquid. There is more than one way to do this.

If the solid sinks, you can pour the liquid off the top. This is called decanting.

If the solid is suspended in the water you can sieve or filter it.

When you sieve the river water, the water goes through the holes in the sieve but the stones can’t fit through the holes. However, the bits of mud are small enough to fit through the holes and so the water is still not pure.

Filter paper has very tiny holes in it. The photograph shows what happens when we put the muddy water through a filter paper. This is called filtering.

The water that we get from filtering the muddy water looks like pure water. However, it is not pure because there are micro-organisms and substances dissolved in the water.

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Water sieving and decanting

Separating Solids and liquids

A solution is a solid dissolved in a liquid. The dissolved solid can’t be removed from a solution by filtering. When the water evaporates from a solution, the solid is left. What is the difference between the solute and the solvent?

Some solids disappear when you put them in water, but the solid hasn’t really disappeared. It has dissolved to form a solution. Everywhere you look in the solution you will find some of the solid. Some solutions are colourless, some are coloured.

How do you get the solid in a solution out again? Because the solid is dissolved in the liquid, it can’t be sieved. The particles are too mixed up and too small. A filter will not work.

To get the dissolved particles of solid back, you may need to remove the water by evaporation.

Remember what happens to a puddle on a warm day? It eventually dries up. The water doesn’t disappear though. It evaporates and turns from a liquid into water vapour that escapes in the air.

If you leave a solution in a warm place the water will evaporate leaving the solid in the bottom of the dish. Instead of removing the solid from the liquid you are removing the water from the solid.

It is important to distinguish between three closely related terms solute, solvent, and solution.

Solute – The substance that dissolves to form a solution

Solvent – The substance in which a solute dissolves

Solution – A mixture of one or more solutes dissolved in a solvent

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Making pure water

When a solution evaporates, the water becomes pure. Only the water evaporates from a solution. What is distillation?

What happens to the liquid when you leave a solution in a warm place? What is left in the bottom of the dish?

If you want to recover the solid that was dissolved in the water, then you leave the solution in a warm place. The water evaporates, leaving the solid behind. But where does the water go? Does some of the solid leave the dish with the water or is it the condensed water pure?

Solid particles are bigger than liquid ones. The solid is too heavy to escape from the solution, so the liquid is the only part that escapes. The water may condense on a cold surface to form pure water.

You get pure water when you evaporate and condense a salt solution.

Evaporation and condensation are part of the natural water cycle. Water evaporates from the sea, and rises into the air. As it rises, it cools and condenses. This turns it into water droplets and clouds. These cool further, grow bigger and fall as rain. This is called the water cycle or the rain cycle.

Car batteries are filled with distilled water. This is water that has been evaporated and then condensed to make sure that nothing is dissolved in it. It is pure water.

Distilled water is made using apparatus similar to that shown in the diagram.

In some countries there is not enough water to drink and so they make drinking water from the sea. Large factories called desalination plants take water out of the sea and heat it up. The steam is condensed and the water is then piped to people’s homes for them to drink and wash with.

Drinking dirty water can cause serious diseases like cholera and typhoid.

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Dissolving sugar

A solution is saturated when no more solid will dissolve. Water can dissolve more of some solids than others. Factors affecting solubility.

People often add sugar to tea to make it taste sweeter. The sugar dissolves in the tea. The amount of solute dissolved in a solvent is affected by:

1. Temperature.2. Stirring.3. Particle size.4. Amount of solute.5. Amount of solvent.

When a solid is added to water and dissolves, the solid particles are mixing up with the liquid particles. Eventually the liquid particles don’t have any space to hold any more solid particles, so no more will dissolve. This is called a saturated solution.

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Graphs and Charts

How can you show your results?

Some children have investigated making things dissolve faster. Their teacher has asked them to draw line graphs or bar charts of their results.

You use a line graph when there are numbers between two measurements that mean something. For example, James used water temperatures of 20, 30 and 40°C but he could have used a temperature between 30 and 40°C (e.g. 35°C). There are numbers between the temperatures he used which mean something, so he can draw a line graph.

Example 1:

In her investigation, Magda changed the brand of the sugar she used. Help her to decide either to use a bar chart or a line graph to represent her results. Plot the graph for her in your notebook.

Brand of sugar Time it takes for the sugar to dissolve (seconds)Pascoe’s 32Kyle and sons 34Brandon 29

Example 2:

James did an experiment about how the amount of sugar affects the time it takes for it to dissolve. Plot the results in your notebook.

Amount of sugar (gm) Time it takes for the sugar to dissolve (seconds)5 2310 4015 6620 84

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Sieves(a) Ahmed uses different sieves to separate different mixtures. His mixtures are made of salt, sugar, rice, dried peas and pasta.

Ahmed has three sieves. Draw THREE lines to match each mixture to the sieve that separates the mixture.

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(b) Ahmed cannot separate a sugar and salt mixture with any of his sieves.Explain why both the sugar and salt go through all Ahmed’s sieves..................................................................................................................

(c) Ahmed mixes salt and water.Salt and water cannot be separated with any sieve.

(i) Explain what happens to the salt when he mixes it with water.......................................................................................................

(ii) Describe how Ahmed could separate a mixture of salt and water.............................................................................................................................................................................................................

(d) Ahmed makes a new mixture of soil and water.

He sieves the mixture. Some of the soil stays in the sieve but some soil goes through the sieve with the water. What is a better way for Ahmed to separate more of the soil from the water quickly?.................................................................................................................

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