Is Matter Around Us Pure

Activity : 2.1 (chapter: Is matter around us pure)

Let us divide the class into groups A, B, C and D.

• Group A takes a beaker containing 50 mL of water and one spatula full of copper sulphate powder. Group B takes 50 mL of water and two spatulae full of copper sulphate powder in a beaker.

• Groups C and D can take different amounts of copper sulphate and potassium permanganate or common salt (sodium chloride) and mix the given components to form a mixture.

• Report the observations on the uniformity in colour and texture.

• Groups A and B have obtained a mixture which has a uniform composition throughout. Such mixtures are called homogeneous mixtures or solutions. Some other examples of such mixtures are (i) salt dissolved in water and (ii) sugar dissolved in water. Compare the colour of the solutions of the two groups. Though both the groups have obtained copper sulphate solution the intensity the colour of the solutions are different. This shows that a homogeneous mixture can have a variable composition.

• Groups C and D have obtained mixtures, which contain physically distinct parts and have non-uniform compositions. Such mixtures are called heterogeneous mixtures. Mixtures of sodium chloride and iron filings, salt and sulphur, and oil and water are examples of heterogeneous mixtures.

Answer

Students of Group (A) took one spatula of copper sulphate and mixed it with 50 ml of water. And group (B) mixed two spatulae of copper sulphate in 50 ml of water.

Since copper sulphate is soluble in water, so the mixture made in this process is a homogeneous mixture. Such mixtures are called solution.

Whereas group (B) dissolved more copper sulphate in the same volume of water, so the colour of the solution is more intense.

A solution having the same composition throughout is known as homogeneous mixture or solution.

Group (C) and (D) both take a different amount of copper sulphate powder and potassium permanganate powder and mix them together.

By mixing copper sulphate and potassium powder together a mixture has been made. In this mixture composition of particles are not even throughout.

This type of mixture in which particles of components are not even throughout is called Heterogeneous mixture

By mixing sodium chloride (common salt) and sugar together, a heterogeneous mixture can be made.

Similarly, many heterogeneous mixtures can be made by mixing different components.

Activity: 2.2 (chapter: Is matter around us pure)

Let us again divide the class into four groups – A, B, C, and D.

• Distribute the following samples to each group:

••Few crystals of copper sulphate to group A.

••One spatula full of copper sulphate to group B.

••Chalk powder or wheat flour to group C.

••Few drops of milk or ink to group D.

• Each group should add the given sample to water and stir properly using a glass rod. Are the particles in the mixture visible?

• Direct a beam of light from a torch through the beaker containing the mixture and observe from the front.

Was the path of the beam of light visible?

• Leave the mixtures undisturbed for a few minutes (and set up the filtration apparatus in the meantime). Is the mixture stable or do the particles begin to settle after some time?

• Filter the mixture. Is there any residue on the filter paper?

Discuss the results and form an opinion.

• Groups A and B have got a solution.

• Group C has got a suspension.

• Group D has got a colloidal solution.

Solution

Question (a) Each group should add the given sample to water and stir properly using a glass rod. Are the particles in the mixture visible?

Answer

For mixture prepared by group (A) and (B)

No, particles of the mixture are not visible.

Explanation

Group (A) and group (B) prepared mixtures of copper sulphate and water.

Since copper sulphate is soluble in water, these groups prepared a homogeneous mixture.

A homogeneous mixture of two or more substances is called solution.

Particles of a homogeneous mixture, i.e. solution are so small that they cannot be seen by naked eyes.

For mixture prepared by group (C)

Yes. Particles of the mixture are visible.

Explanation

Group (C) prepared a mixture of chalk powder or wheat flour and water.

Chalk powder or wheat flour is not soluble in water and settles down after some time.

Such a mixture of two or more substances is called a suspension.

A suspension is a heterogeneous mixture of two or more substances.

The particles of a suspension are big enough to see from naked eyes.

For mixture prepared by group (D)

No. Particles of the mixture are not visible.

Explanation

Group (D) prepared a mixture of milk or ink and water.

This type of mixture is called colloidal or colloidal solution.

A colloidal is a heterogeneous mixture of two or more substances.

A colloidal is appeared to be homogeneous but actually, it is a heterogeneous mixture.

The particles of a colloidal are relatively bigger in size compared to a solution, but cannot be seen through naked eyes.

Question (b) Direct a beam of light from a torch through the beaker containing the mixture and observe from the front.

Was the path of the beam of light visible?

Answer

For mixture prepared by group (A) and (B)

No, the path of the beam of light is not visible.

Explanation

Group (A) and (B) prepared a mixture of copper sulphate and water. This is called a solution.

The particles of solutions are very small in size, so they do not scatter the path of the beam of light, because of this the path of the beam of light is not visible through a solution.

For mixture prepared by group (C)

Answer

Yes. The path of the beam of light is visible.

Explanation

Group (C) prepared a mixture of wheat flour or chalk powder and water.

This type of mixture is called a suspension.

The particles of a suspension are bigger enough to scatter the beam of light that's why the path of the beam of light is visible through a suspension.

For mixture prepared by group (D)

Answer

Yes. The path of the beam of light is visible.

Explanation

Group (D) prepared a mixture of milk or ink and water. This type of mixture is called a colloidal or colloidal solution.

The particles of a colloidal solution are called colloids. The colloids are big enough to scatter the beam of light that's why the path of the beam of light is visible through a colloidal solution.

Question (c) Leave the mixtures undisturbed for a few minutes (and set up the filtration apparatus in the meantime).

Is the mixture stable or do the particles begin to settle after some time?

Answer

For the mixture prepared by group (A) and (B)

The mixture is stable and particles of the mixture do not settle when the mixture is left undisturbed.

Explanation

Groups (A) and (B) prepared a homogeneous solution. A solution is stable. In a homogeneous solution, the particles are distributed throughout evenly and do not settle down.

For the mixture prepared by group (C)

The mixture is not stable. The particles of a mixture prepared by group (C) begin to settle when the mixture is left undisturbed for some time.

Explanation

The mixture of chalk powder or flour and water prepared by group (C) is called a suspension. Suspension is a heterogeneous mixture of two or more substances.

A suspension is unstable. The particles of a suspension start settling down when left disturbed for some time.

For the mixture prepared by group (D)

No. The mixture is stable and particles of the mixture prepared by group (D) do not begin to settle down when left undisturbed.

Explanation

The mixture prepared by group (D) is called a colloidal or colloidal solution. A colloidal is stable and the particles of a colloidal solution do not settle down when left undisturbed.

Question (d) Filter the mixture. Is there any residue on the filter paper?

Answer

For the mixture prepared by group (A) and (B)

No. There is no residue left on the filter paper when the mixture prepared by groups (A) and (B) is filtered.

Explanation

The mixture prepared by the group (A) and (B) is called solution. A solution is a homogeneous mixture of two or more substances, i.e. particles are distributed evenly throughout and particles of a solution are so small that they cannot be filtered using a filter paper.

For the mixture prepared by group (C)

Yes, the residue is left on the filter paper when the mixture prepared by group (C) is filtered.

Explanation

The mixture prepared by group (C) is called a suspension. A suspension is unstable. The particles of a suspension are bigger enough so that they can be filtered using filter paper. Thus, a residue is obtained on the filter paper when the mixture prepared by group (c) is filtered.

For the mixture prepared by group (D)

No residue is left on the filter paper when the mixture prepared by group (D) is filtered.

Explanation

The mixture prepared by group (D) is called a colloidal. The colloids are distributed evenly throughout the mixture and appeared to be homogeneous, but a colloidal is a heterogeneous mixture of two or more substances.

The particles of a colloidal are big enough to scatter the path of the beam of light but too small to be filtered.