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

Ask Yourself

Question 1. How can we conclude that air is a matter?
Answer:
Air occupies space: a balloon or a tyre becomes bigger when air is filled in it.
Air has mass: a balloon filled with air weighs more than the same balloon when empty.
Anything that has mass and occupies space is matter, so air is matter.
In simple words: Air fills a balloon and makes it heavier, so air is matter.

Teacher's Note:
1. Use both properties: mass and space.
2. You cannot see air, but its effects show it is matter.

Question 2. Explain why cold cannot be considered as matter?
Answer:
Cold is not a substance. It is only a feeling, caused by a low temperature, or the absence of heat.
It has no mass and it does not occupy space, so it is not matter.
In simple words: Cold has no mass and takes up no space.

Teacher's Note:
1. Light, heat and sound are also not matter.
2. Check the two properties of matter.

Question 3. Explain how mixture of salt and water is homogeneous mixture?
Answer:
When salt is stirred in water, it dissolves completely and its particles cannot be seen.
The salt is spread evenly, so the first sip tastes as salty as the last.
The composition is the same throughout, so the mixture is homogeneous.
In simple words: Salt spreads evenly in water, so every part tastes the same.

Teacher's Note:
1. Use the words 'uniform composition'.
2. A taste test is a simple proof of uniformity.

Question 4. Explain heterogeneous mixture with two examples.
Answer:
A heterogeneous mixture is one that does not have a uniform composition throughout; its parts can be seen and often settle.
Examples: mud in water (the soil particles settle down) and oil and water (the oil floats as a separate layer).
In simple words: In a heterogeneous mixture the different parts can be seen.

Teacher's Note:
1. Soil and a mixture of sand and water are other examples.
2. Always give the two examples asked.

Question 5. Define the terms handpicking and threshing by giving examples.
Answer:
Handpicking: separating the parts of a mixture by picking them by hand, when they differ in size, shape or colour. Example: removing stones from rice.
Threshing: separating grains from the harvested stalks by beating the stalks. Example: freeing wheat grains from wheat stalks, by hand, bullocks or a machine.
In simple words: Handpicking is picking out pieces by hand; threshing is beating stalks to get the grain.

Teacher's Note:
1. Give one example for each term.
2. Threshing is done before winnowing.

Question 6. How a mixture of iron filing and sulphur powder can be separated?
Answer:
Spread the mixture and move a magnet over it. Iron is magnetic, so the iron filings stick to the magnet.
Sulphur is not magnetic, so it stays in the dish.
Remove the filings from the magnet and repeat until no iron remains. This is magnetic separation.
In simple words: A magnet pulls out the iron and leaves the sulphur.

Teacher's Note:
1. Only one of the two parts is magnetic.
2. Brushing the filings off the magnet into a separate pile finishes the job.

Question 7. Which property among ammonium chloride and sodium chloride is different?
Answer:
Ammonium chloride sublimes: it changes directly from solid to vapour on heating.
Sodium chloride does not sublime. So a mixture of the two can be separated by sublimation.
In simple words: Ammonium chloride sublimes on heating, sodium chloride does not.

Teacher's Note:
1. This difference lets us separate them.
2. Camphor and iodine also sublime.

Question 8. Why water droplets are seen on the surface of water bottle containing cold water?
Answer:
There is water vapour in the air around the bottle.
When this vapour touches the cold surface of the bottle it cools and changes into liquid water.
This process is condensation. The water does not come from inside the bottle.
In simple words: Water vapour from the air condenses on the cold bottle.

Teacher's Note:
1. The droplets are on the outside, not leaking.
2. Name the process: condensation.

Question 9. Define the principle of centrifugation technique.
Answer:
When an object is spun at high speed, it experiences an outward force, away from the centre of rotation.
In centrifugation, a mixture in a tube is spun fast; the heavier solid particles are thrown outwards to the bottom of the tube, and the clear liquid stays on top.
In simple words: Fast spinning pushes heavier particles outwards to the bottom of the tube.

Teacher's Note:
1. Start with the outward force.
2. Mention the centrifuge machine.

Question 10. Write two applications of centrifugation technique.
Answer:
1. Separating cream from milk in a dairy.
2. Drying wet clothes in the spinner of a washing machine, where water is thrown out through the holes of the drum.
In simple words: Cream is got from milk and clothes are dried by spinning.

Teacher's Note:
1. Other uses include separating blood parts in a laboratory.
2. Give two different examples.

Question 11. Why separating funnel is not implemented for separation of two miscible liquid?
Answer:
Miscible liquids mix completely and make one single layer.
A separating funnel works only when the liquids form two separate layers, so that the lower layer can be drained through the tap.
Miscible liquids have no such layers, so they are separated by distillation instead.
In simple words: Miscible liquids do not form layers, so there is nothing to drain separately.

Teacher's Note:
1. Layers form only for immiscible liquids.
2. Distillation is used for miscible liquids.

Question 12. How the mixture of benzene, water and mercury can be separated?
Answer:
The three liquids do not mix with each other, so they form three layers in a separating funnel: mercury (very dense, about 13.6 g per cm³) at the bottom, water in the middle and benzene (about 0.88 g per cm³) on top.
Open the stopcock and run out the mercury first and close the tap. Run the water into another beaker next.
Benzene is left in the funnel.
In simple words: Pour the three liquids into a separating funnel and drain them one by one, starting from the heaviest.

Teacher's Note:
1. Arrange the liquids by density.
2. Drain from the bottom upwards.

Question 13. Explain the term solute and solvent.
Answer:
Solute: the substance that dissolves in a liquid, for example salt.
Solvent: the liquid in which the solute dissolves, for example water.
Together they make a solution (salt solution).
In simple words: The solute dissolves; the solvent is the liquid that dissolves it.

Teacher's Note:
1. Salt in water: salt is the solute, water is the solvent.
2. Solute plus solvent makes the solution.

Teacher's Note:
1. A table of differences scores well.
2. Supersaturated solutions need heating.

Teacher's Note:
1. Mention that temperature stays the same.
2. Heating can turn a saturated solution into an unsaturated one.

Question 16. How will you find that the solution is super saturated?
Answer:
Prepare a saturated solution and warm it, so that a little more solute dissolves in it. This solution is supersaturated.
When it cools, the extra solute separates out and appears at the bottom of the beaker. Seeing this extra solute settle on cooling shows that the solution was supersaturated.
In simple words: A supersaturated solution drops the extra solute when it cools.

Teacher's Note:
1. Heating is needed to make it.
2. Cooling is how we confirm it.

Short Answer Type

Question 17. Give any four example of mixtures found on a kitchen shelf.
Answer:
1. Garam masala (a mixture of many spices).
2. Mixed pulses (different kinds of dal together).
3. Tea leaves mixed with tea dust in a tea packet.
4. Rice mixed with small stones or husk before cleaning (or mixed dry fruits).
In simple words: Many things we store in the kitchen are mixtures of two or more materials.

Teacher's Note:
1. Name the parts of each mixture too.
2. Pure salt and sugar are not mixtures.

Question 18. Why do we need to separate different components of a mixture?
Answer:
1. To get two useful things from a mixture, for example butter from milk.
2. To remove harmful parts or impurities, for example stones and husk from rice or dal.
3. To remove a part that is of no use, for example tea leaves from tea.
In simple words: We separate mixtures to get useful parts and remove harmful or useless ones.

Teacher's Note:
1. Give one example for each reason.
2. Three reasons are expected.

Question 19. Why is loading used to separate suspended impurities?
Answer:
Very tiny insoluble particles stay floating in water for a long time and settle very slowly.
In loading, alum is added to the water. It makes the tiny particles join together and become heavy, so they sink quickly.
The clear water can then be decanted. Loading is used to get clear water from muddy water.
In simple words: Alum makes tiny floating particles heavy so they settle quickly.

Teacher's Note:
1. Name alum; do not forget it.
2. Loading is followed by decantation.

Question 20. What is evaporation? How is it useful to us?
Answer:
Evaporation is the change of a liquid, such as water, into vapour at a temperature below its boiling point.
Uses: (1) common salt is obtained from sea water by evaporating the water in the sun; (2) dissolved solids such as sugar are recovered from their solutions; (3) wet clothes dry because the water in them evaporates.
In simple words: Evaporation is liquid changing to vapour below the boiling point; we use it to get salt and dry clothes.

Teacher's Note:
1. Give at least two uses.
2. Evaporation takes place at any temperature.

Question 21. Name any two methods, which are used to separate: (a) Solid-solid mixtures (b) Liquid-solid mixtures
Answer:
(a) Solid-solid mixtures: handpicking and winnowing (also threshing, sieving, magnetic separation and sublimation).
(b) Liquid-solid mixtures: filtration and sedimentation with decantation (also loading, evaporation and centrifugation).
In simple words: Pick any two methods from each group.

Teacher's Note:
1. Match the method to the state of the mixture.
2. The concept map in the notes lists all of them.

Conceptual Questions

Question 22. Why crystallisation is a better technique than evaporation?
Answer:
During evaporation the solution is heated to dryness. Some solids break down on this heating, and some, like sugar, get charred.
Soluble impurities remain in the solution even after filtering. On evaporation they dry along with the solid and make it impure.
In crystallisation the solution is allowed to cool and stand, so pure crystals form and the impurities stay in the solution.
In simple words: Crystallisation gives pure crystals and does not burn the solid.

Teacher's Note:
1. Give both reasons.
2. Crystals leave impurities behind.

Question 23. Explain why fractional distillation but not simple distillation is used to separate a mixture of two miscible liquids whose boiling point differ by about 25°C.
Answer:
When the boiling points are close, both liquids give vapour as the mixture is heated, so simple distillation gives a distillate that is a mixture of the two.
A fractionating column placed above the flask has many surfaces on which the vapours cool and evaporate again and again. The vapour of the liquid with the lower boiling point rises further, and the other liquid returns to the flask.
So only the liquid with the lower boiling point reaches the condenser first, and the two liquids are separated properly.
In simple words: A fractionating column gives a cleaner separation when the boiling points are close.

Teacher's Note:
1. Say what the column does.
2. Simple distillation works for liquids with a big boiling point gap.

Question 24. The boiling point of liquid oxygen and liquid nitrogen are -183°C and -196°C respectively. Which will evaporate first, liquid oxygen or liquid nitrogen?
Answer:
Liquid nitrogen boils at -196°C, which is lower than the boiling point of liquid oxygen (-183°C).
On warming the mixture, the liquid with the lower boiling point turns to gas first. So liquid nitrogen will evaporate first.
In simple words: The liquid with the lower boiling point evaporates first, and that is nitrogen.

Teacher's Note:
1. -196 is a lower temperature than -183.
2. This difference is used in fractional distillation of liquid air.