Aseel Samaro

Modelling mixtures and separation

Architects use drawings and models to show how new buildings might look.

These models are smaller, simpler copies of the real thing.

Similarly, we can use a scientific model to help explain observations in science.

Introduction

A circle model helps us to understand the composition of substances.

Each circle represents an atom.

Different colours represent different atoms.

When atoms bond together they form a molecule.

If a molecule contains different types of atom it is called a compound.

If a substance only contains one type of atom or molecule it is pure.

Mixtures can contain any number or type of different atoms or molecules.

Circle model of substances

Atoms and molecules represented by the circle model

Why are circle models useful?

Draw circle models to represent:one atom

a molecule consisting of two identical atomsa molecule consisting of two different atoms

a mixture of different molecules.

Why are circle models useful?Circle models help scientists to understand the composition of substances, explain their

observations and explain processes simply, or explain things that cannot be seen directly

Draw circle models to represent:one atom

a molecule consisting of two identical atomsa molecule consisting of two different atoms

a mixture of different molecules.

When salt dissolves in water the crystals seem to disappear, but they are still there, mixed with the water molecules.

This can be proved by measuring the mass of the substances.

No atoms are lost and no new atoms are added.

This is called the Law of Conservation of Mass.

Conservation mass

The mass of water and salt on the first balance is the same as the mass of the mixture on the second balance.

What is a solution?

How does the mass of a solution show that the solute has not disappeared?

Draw diagrams, using the circle model, of a) salt and water, b) salt dissolved in water, to illustrate the Law of Conservation of Mass.

The same experiment was repeated, but this time the mass of the solution was lower than the original mass of the salt and water. What might have

happened?

What is a solution?a mixture of a solvent and a solute

How does the mass of a solution show that the solute has not disappeared?It has the same total mass as the solute and solvent together/there is no change in mass.

Draw diagrams, using the circle model, of a) salt and water

b) salt dissolved in water, to illustrate the Law of Conservation of Mass.Two circle models needed:

a) Separate solvent and solute diagrams; solvent circles spread out; solute circles clustered more tightly (can be atoms or molecules; single or groups of circles).

b) The same number of circles as the solute and solvent in (a); in one diagram; randomly mixed together

The same experiment was repeated, but this time the mass of the solution was lower than the original mass of the salt and water. What might have happened?

Some of the salt or water was spilled; some water evaporated.

In filtration, the filtrate particles are drawn smaller to show that it can pass through the filter paper.

Different colours are used to represent the different substances. The filtrate is a mixture of the smaller particles of two substances.

Models simplify a process to help explain how things work. You do not always need to see all the detail to understand what is happening.

Modelling

This model shows dissolving.A model of filtration

In what ways is the model shown in Figure 1.3.16d a good one for dissolving?

How could this model be improved?Draw a model to show:

evaporation of water from salt solutionseparation of iron filings from sand.

In what ways is the model shown in Figure 1.3.16d a good one for dissolving?The salt particles get smaller; and mix with the water particles.

How could this model be improved?The water particles could be smaller; there could be more of them; could imply movement; salt

particles could show that they are made of lots of particles stuck together.

Draw a model to show: evaporation of water from salt solution A model similar to that shown in Figure 1.3.16d with some

of the smaller water circles above the level of the solution.

separation of iron filings from sand. A model to show a magnet with iron ‘circles’ attached; and sand ‘circles’ still in a pile.

Chocolate is a mixture.

The ratio of cocoa, sugar, milk and fat is carefully controlled and blended so that the chocolate melts in your mouth at the right temperature.

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