CBSE Class 6 Science Advanced Chemistry Matter and Its Segregation Long Answer Questions Set 01

Long Answer Type

Question 1. Why is salt a pure substance whereas salt solution is considered to be a mixture?
Answer:
Salt (sodium chloride) contains only one kind of particle and it has fixed properties, such as a definite melting point. So salt is a pure substance (a compound).
A salt solution contains two different substances, salt and water, mixed together. The amount of salt in it can be changed (a little salt or a lot of salt), and the parts can be separated by a physical method such as evaporation.
A substance whose composition is not fixed and which can be separated into parts is a mixture. Hence salt is pure but salt solution is a mixture.
In simple words: Salt has one kind of particle; salt solution has salt and water in any amount.

Teacher's Note:
1. Compare the number of kinds of particles in each.
2. Mention evaporation as proof that the parts can be separated.

Question 2. Name and describe briefly a method which can be helpful in separating a mixture of husk from grains. What is the principle of this method?
Answer:
The method is winnowing.
Description: the mixture of grain and husk is poured slowly from a height, standing on a platform, in a breeze or in the wind. The light husk is blown away to one side while the heavier grain falls straight down and forms a heap near the platform. The husk is later used as fodder.
Principle: it is based on the difference in weight (density) between the two parts. Moving air carries the lighter part further than the heavier part.
In simple words: Pour the mixture in the wind; light husk blows away and heavy grain falls down.

Teacher's Note:
1. State the principle separately; marks are given for it.
2. Add that threshing is done before winnowing.

Question 3. Draw a well-labelled diagram for the setup used for filtration. Explain its working.
Answer:

Working: a filter paper is folded into a cone and placed in a funnel, which is held over a beaker. The mixture, for example sand and water, is poured slowly along a glass rod into the funnel.
The water passes through the tiny pores of the filter paper into the beaker and is called the filtrate. The sand, being insoluble, is too big to pass through and stays on the paper as the residue.
Dissolved substances, such as salt, would go through the paper with the water.
In simple words: A filter paper in a funnel lets the liquid through and holds the solid back.

Teacher's Note:
1. Label funnel, filter paper, residue and filtrate.
2. Pour along a glass rod to avoid splashing.

Question 4. Name the method of separation that can be used in the following situations. (a) Accidental mixing of mustard oil and water. (b) Adulterated urad dal having papaya seeds in it. (c) You visit a village where the water drawn from the well is not transparent. (d) If you find that your bhel puri has large number of cut green chilies.
Answer:
(a) A separating funnel (the oil floats and the water is drained from below). Decantation is also possible.
(b) Handpicking, because the papaya seeds differ in size and colour from the dal.
(c) Loading (adding alum) followed by sedimentation and decantation, or filtration, to get clear water.
(d) Handpicking, to take the green chillies out by hand.
In simple words: Choose the method by checking the type of mixture and the property that differs.

Teacher's Note:
1. Decide first whether the mixture is solid-solid, solid-liquid or liquid-liquid.
2. Give the method for each part.

Teacher's Note:
1. Use a table for each part.
2. Give an example in each case.

Question 6. Lemonade is made by mixing lemon juice and sugar in water. You wish to add ice to cool it. Should you add ice to the lemonade before or after dissolving sugar? In which case would it be possible to dissolve more sugar?
Answer:
Ice should be added after dissolving the sugar.
Cold water can dissolve less sugar than water at room temperature. If ice is added first, the lemonade becomes cold and the sugar dissolves less well, and the solution may become saturated soon.
If the sugar is dissolved first in the water at normal temperature, more sugar dissolves. The ice is then added to cool the sweet lemonade.
So more sugar can be dissolved when ice is added after dissolving the sugar.
In simple words: Dissolve the sugar first, since cold water takes up less sugar.

Teacher's Note:
1. Solubility of a solid usually goes up on heating and down on cooling.
2. Link the answer to the saturated solution.