This chapter for Class 6 Chemistry basically introduces the idea that some stuff, called pure substances (either simple elements or combined compounds), are made of just one kind of “building block”
Then, it talks about mixtures, which are just different things mixed together without any chemical bonding. These mixtures can be homogeneous (looking the same all over) or heterogeneous (where you can see the different parts).
Finally, the chapter explains how to separate these mixtures using simple physical tricks like letting things settle, pouring off liquids carefully, using filters, boiling off liquids, or even using magnets or sieves – all based on the different properties of what’s mixed together. It’s all about knowing what things are made of and how to unmix them!
EXERCISE – I
Question 1.
Select homogeneous and heterogeneous mixtures from the following:
Salt solution, petrol and water, sand and charcoal, alcohol and water, air dissolved in water, air, sea water, fruit juices, mist, brass.
Answer:Homogeneous mixture : Salt solution, alcohol and water, air dissolved in water, sea water, brass.
Heterogeneous mixture: Sand and charcoal, air, fruit juice, mist, petrol and water.
Question 2.
Define the following :
(a) Pure substance
(b) Impure substance
(c) Alloy
(d) Solution
(e) Heterogeneous mixture
(f) Homogeneous mixture
Answer:(a) Pure substance: Imagine something made of only one kind of stuff – like pure water (H2O) or gold (Au). It has a consistent composition throughout.
(b) Impure substance: This is when you’ve got a mix of different kinds of stuff together, not chemically bonded. Think of soil, which has bits of rock, sand, and other things mixed in.
(c) Alloy: This is a special kind of impure substance – it’s a mixture of two or more metals (or a metal and a nonmetal) that are melted together. Steel, a mix of iron and carbon, is a common example.
(d) Solution: Picture sugar dissolved in water. You can’t see the sugar anymore, right? That’s a solution – a type of mixture where one substance dissolves completely into another, making it look uniform throughout.
(e) Heterogeneous mixture: This is a mixture where you can still see the different parts. Think of a salad – you can clearly see the lettuce, tomatoes, and cucumbers. The composition isn’t the same everywhere.
(f) Homogeneous mixture: This is a mixture that looks the same all the way through, like that sugar water we talked about. You can’t pick out the different components with your eye. The composition is uniform.
Question 3.
List four characteristics of a mixture.
Answer:Mixtures show these key traits:
- They contain two or more substances physically combined.
- Their composition isn’t fixed; you can have varying amounts of each substance.
- They can usually be separated by physical means.
Question 4.
Give reasons :
(a) Why do sugar and water retain their individual properties in a sugar solution ?
(b) Why do petrol and water form a heterogeneous mixture ?
(c) Why does sulphur dissolve when carbon disulphide is added to a mixture of iron and sulphur but not when it is added to iron sulphide ?
Answer:(a) In a sugar solution, The sugar molecules are dispersed among the water molecules, but no chemical reaction occurs to change their fundamental nature.
(b) Due to these differences in polarity, the molecules of petrol and water do not attract each other strongly enough to mix at a molecular level, resulting in distinct layers.
(c) Sulphur dissolves in carbon disulphide because both are nonpolar substances, and “like dissolves like.” In a mixture of iron and sulphur, the sulphur exists in its elemental form and can interact with the carbon disulphide. However, in iron sulphide, sulphur has chemically reacted with iron to form a new compound with different properties, and it no longer exists as elemental sulphur that can dissolve in carbon disulphide.
Question 5.
Give two examples for each of the following types of mixture.
(a) solid-solid
(b) solid-liquid
(c) liquid-gas
(d) gas-gas
Answer:a) Solid-Solid: Think granite – different solid bits stuck together. Or brass, where you melt solids (copper and zinc) and they mix into a new solid.
(b) Solid-Liquid: Sugar in water disappears but it’s there – dissolved. Sand in water just hangs out or sinks; it doesn’t dissolve.
(c) Liquid-Gas: Fizzy drinks have gas (like carbon dioxide) mixed into the liquid, making bubbles. Humid air is just air (gas) with tiny water drops (liquid) making it feel wet.
(d) Gas-Gas: Air is the big one – a mix of gases like nitrogen and oxygen. Car exhaust is another example, a blend of different gases.
Question 6.
Name the components present in the following mixtures:
(a) Brass
(b) Duralumin
(c) Tap water
(d) Bronze
(e) Crude petroleum oil .
Answer:(a) Brass: Copper and Zinc
(b) Duralumin: Aluminum, Copper, Magnesium, and Manganese
(c) Tap water: Water and dissolved salts (and sometimes other substances like minerals or chlorine)
(d) Bronze: Copper and Tin
(e) Crude petroleum oil: A complex mixture of various hydrocarbons (like alkanes, alkenes, alkynes, and aromatic compounds).
Question 7.
State:
(a) Three differences between water and air.
(b) Four differences between compounds and mixtures.
Answer:
Question 6.
Name the components present in the following mixtures:
(a) Brass
(b) Duralumin
(c) Tap water
(d) Bronze
(e) Crude petroleum oil .
Answer:(a) Brass: Copper and Zinc
(b) Duralumin: Aluminum, Copper, Magnesium, and Manganese
(c) Tap water: Water and dissolved salts (and sometimes other substances like minerals or chlorine)
(d) Bronze: Copper and Tin
(e) Crude petroleum oil: A complex mixture of various hydrocarbons (like alkanes, alkenes, alkynes, and aromatic compounds).
Question 7.
State:
(a) Three differences between water and air.
(b) Four differences between compounds and mixtures.
Answer:
| Feature | Water (Pure) | Air (Primarily a Mixture) |
| Composition | A chemical compound with a fixed ratio of hydrogen and oxygen (H₂O). | A physical mixture of gases, primarily nitrogen (N₂), oxygen (O₂), argon (Ar), carbon dioxide (CO₂), and trace amounts of others. The ratio of these gases can vary slightly. |
| State at Room Temperature | Typically a liquid under standard conditions (around 25°C and 1 atm pressure). Can exist as a solid (ice) or gas (steam) under different temperatures and pressures. | Typically a gas under standard conditions. |
| Compressibility | Relatively incompressible. It’s difficult to significantly reduce its volume by applying pressure. | Highly compressible. Its volume can be significantly reduced by applying pressure. |
(b)
| Feature | Compounds | Mixtures |
| Composition | Made up of two or more elements chemically combined in a fixed ratio by mass. | Made up of two or more substances (elements or compounds) physically mixed together in any ratio. |
| Separation of Components | Components cannot be separated by simple physical means (like filtration, evaporation, magnetism). Chemical reactions are required to break them down. | Components can usually be separated by simple physical means that exploit differences in their physical properties (e.g., filtration, evaporation, distillation, magnetism). |
| Properties | Have properties that are entirely different from the properties of their constituent elements. They behave as a single substance. | Show the properties of their constituent substances to some extent. The overall properties are often an average of the properties of the components. |
| Melting and Boiling Points | Have sharp and definite melting and boiling points under given pressure. | Usually have a range of melting and boiling points, as the composition can vary. |
EXERCISE – II
Question 1.
Define:
(a) Filtration
(b) Sublimation
(c) Evaporation
(d) Crystallisation
(e) Miscible liquids
(f) Immiscible liquids
Answer:
(a) Filtration: Like using a kitchen strainer to keep the bits out of your juice. It separates solids from liquids or gases by letting the liquid or gas pass through tiny holes that the solid stuff can’t.
(b) Sublimation: Think of those special air fresheners that disappear over time without leaving any wetness – that’s a solid turning directly into a gas.
(c) Evaporation: It’s just when a liquid slowly turns into a gas, like when rainwater vanishes from the pavement on a sunny day.
(d) Crystallisation: Imagine making rock candy; that’s crystallisation – getting solid, organized bits (crystals) to form out of a liquid.
(e) Miscible liquids: These are liquids that are happy to mix together perfectly, like when you stir sugar into your tea and it all becomes one liquid.
(f) Immiscible liquids: These are the opposite; they just won’t mix and will always separate into layers, like oil floating on top of water in a salad dressing.
Question 2.
Why do we need pure substances?
Answer:
This reliability is key for precise measurements, dependable chemical reactions without unexpected results, and safe use in medications. Their uniformity ensures consistent and trustworthy performance across different uses.
Question 3.
Give one example for each of the following types of mixtures.
(a) Solid-solid heterogeneous mixture
(b) Solid-liquid heterogeneous mixture
(c) Solid-liquid homogeneous mixture
Answer:
(a) Sand and sugar: You can clearly see the separate bits of sand and sugar mixed together.
(b) Muddy water: You can see the soil floating around in the water.
(c) Saltwater: Once the salt dissolves, it looks like just plain water.
Question 4.
Name the process by which the components of following mixtures can be separated.
(a) Powdered glass and sugar
(b) Chalk powder and iron filings
(c) Chaff and grain
(d) Salt and water
(e) Wheat and sugar
(f) Sand and camphor
(g) Sugar and water
Answer:
(a) Powdered glass and sugar: Sieving (followed by perhaps winnowing if particle sizes are significantly different after sieving).
(b) Chalk powder and iron filings: Magnetic separation.
(c) Chaff and grain: Winnowing.
(d) Salt and water: Evaporation (or distillation).
(e) Wheat and sugar: Sieving (if particle sizes are notably different).
(f) Sand and camphor: Sublimation.
(g) Sugar and water: Evaporation (or distillation).
Question 5.
Name:
(a) two substances which can sublime
(b) two substances soluble in water
(c) two substances insoluble in water
(d) four substances that can be used as filters.
Answer:
a) Camphor and naphthalene readily sublime, transitioning directly from solid to gas.(b) Sugar and salt dissolve in water without much trouble.
(c) Sand and chalk don’t mix with water; they stay separate.
(d) You can use cotton, charcoal, filter paper, or fine cloth to separate things.
Question 6.
Give reasons :
(a) Sand and saw dust cannot be separated by hand picking.
(b) Magnet is used to separate a mixture of iron and sulphur.
(c) Alum is used in purification of river water.
Answer:
(a) Separating tiny, mixed particles like sand and sawdust by hand? That sounds like a super tedious job because they’re so small and all mixed up!
(b) Got iron and sulfur mixed? No sweat! Just use a magnet. The iron will cling to it, leaving the sulfur behind.
(c) Dealing with muddy river water? Try adding some alum. It’ll clump all the tiny mud particles together so they sink to the bottom, giving you clearer water up top.
OBJECTIVE TYPE QUESTIONS
1. Fill in the blanks
(a) The substances that make a mixture are called its constituents or components.
(b) Evaporation or crystallisation is a process to separate solids dissolved in liquids.
(c) Mist is a heterogeneous (liquid in gas) mixture of droplets of water and air.
(d) Clay is separated from water by the method of loading and decantation.
(e) When cereals are washed before cooking, water is separated from the cereals by decantation.
(f) Crystallisation is a process to obtain a very pure form of a solid dissolved in a liquid.
(g) Ammonium chloride can be separated from common salt by the method of sublimation.
(h) The solid particles which remain on the filter paper are called residue and the liquid which passes through it is called filtrate.
(i) The process of transferring the clear liquid layer above the solid particles which settle at the bottom of the container is known as decantation.
(j) Filtration is a method used for the separation of an insoluble solid from a solid-liquid mixture.
2. Write “true” or “false” for the following statements
(a) A pure substance consists of only one kind of atom or molecule.
Answer. True
(b) Common salt is separated from its solution in water by decantation.
Answer. False
Correct : Common salt is separated from its solution in water by evaporation.
(c) Winnowing is a process to remove small stones from grains.
Answer. False
Correct : Winnowing is a process to remove husk from grains.
(d) Jewellery gold is a homogeneous mixture of metals.
Answer. False
Correct : Jewellery gold is a heterogeneous mixture of metals.
(e) Air can be separated from water by filtration.
Answer. False
Correct : Air can be separated from water by heating.
(f) Salt and air dissolved in water add taste to water.
Answer. True
(g) Steel is an alloy of iron and aluminium.
Answer. False
Correct: Steel is an alloy of iron and carbon.
MULTIPLE CHOICE QUESTIONS
Tick (√) the correct alternative from the choice given for the following statements:
1. The process of adding a chemical substance to help the suspended solid particles to deposit as sediment fastly is called
- loading
- sedimentation
- decantation
- Filtration
Answer. loading
2. Salt is separated from sea water by
- evaporation
- sublimation
- crystallisation
- Filtration
Answer. evaporation
3. A mixture of mustard oil and water forms
- a compound
- a homogeneous mixture
- an alloy
- a heterogeneous mixture
Answer. a heterogeneous mixture
4. A heterogeneous mixture is
- made up of only one kind of atom
- made up of only one kind of molecule
- made up of different kinds of atoms and molecules.
- that looks uniform
Answer. that looks uniform
5. Example of a homogeneous mixture is
- distilled water
- tap water
- sand and water
- sawdust and water
Answer. tap water
6. A set of mixture is
- gold, common salt, water, alloy
- alloy, ink, honey, icecream
- alloy, mercury, air, sea water
- milk, duralumin, brass, silver
Answer. alloy, ink, honey, icecream
7. A gas dissolved in a liquid can be separated by :
- filtration
- boiling
- using magnet
- by crystallisation
Answer. boiling
8. Copper is not a part of the alloy :
- brass
- bronze
- steel
- Duralumin
Answer. steel
9. Which is not a mixture?
- sugar solution
- tap water
- milk
- distilled water
Answer.
OBJECTIVE TYPE QUESTIONS
1. Fill in the blanks
(a) The substances that make a mixture are called its constituents or components.
(b) Evaporation or crystallisation is a process to separate solids dissolved in liquids.
(c) Mist is a heterogeneous (liquid in gas) mixture of droplets of water and air.
(d) Clay is separated from water by the method of loading and decantation.
(e) When cereals are washed before cooking, water is separated from the cereals by decantation.
(f) Crystallisation is a process to obtain a very pure form of a solid dissolved in a liquid.
(g) Ammonium chloride can be separated from common salt by the method of sublimation.
(h) The solid particles which remain on the filter paper are called residue and the liquid which passes through it is called filtrate.
(i) The process of transferring the clear liquid layer above the solid particles which settle at the bottom of the container is known as decantation.
(j) Filtration is a method used for the separation of an insoluble solid from a solid-liquid mixture.
2. Write “true” or “false” for the following statements
(a) A pure substance consists of only one kind of atom or molecule.
Answer : True
(b) Common salt is separated from its solution in water by decantation.
Answer. False
Correct : Common salt is separated from its solution in water by evaporation.
(c) Winnowing is a process to remove small stones from grains.
Answer. False
Correct : Winnowing is a process to remove husk from grains.
(d) Jewellery gold is a homogeneous mixture of metals.
Answer. False
Correct : Jewellery gold is a heterogeneous mixture of metals.
(e) Air can be separated from water by filtration.
Answer. False
Correct : Air can be separated from water by heating.
(f) Salt and air dissolved in water add taste to water.
Answer. True
(g) Steel is an alloy of iron and aluminium.
Answer. False
Correct: Steel is an alloy of iron and carbon.
MULTIPLE CHOICE QUESTIONS
Tick (√) the correct alternative from the choice given for the following statements:
1. The process of adding a chemical substance to help the suspended solid particles to deposit as sediment fastly is called
- loading
- sedimentation
- decantation
- Filtration
Answer : loading
2. Salt is separated from sea water by
- evaporation
- sublimation
- crystallisation
- Filtration
Answer : evaporation
3. A mixture of mustard oil and water forms
- a compound
- a homogeneous mixture
- an alloy
- a heterogeneous mixture
Answer: a heterogeneous mixture
4. A heterogeneous mixture is
- made up of only one kind of atom
- made up of only one kind of molecule
- made up of different kinds of atoms and molecules.
- that looks uniform
Answer: that looks uniform
5. Example of a homogeneous mixture is
- distilled water
- tap water
- sand and water
- sawdust and water
Answer : tap water
6. A set of mixture is
- gold, common salt, water, alloy
- alloy, ink, honey, icecream
- alloy, mercury, air, sea water
- milk, duralumin, brass, silver
Answer : alloy, ink, honey, icecream
7. A gas dissolved in a liquid can be separated by :
- filtration
- boiling
- using magnet
- by crystallisation
Answer: boiling
8. Copper is not a part of the alloy :
- brass
- bronze
- steel
- Duralumin
Answer: steel
9. Which is not a mixture?
- sugar solution
- tap water
- milk
- distilled water
Answer : distilled water
10. Give one word name for the following
(a) The solid which is left on the filter paper after filtration
Ans : Residue.
(b) The solid particles which separate out from the solution on slow evaporation
Ans : Crystal
(c) The solid particles that settle at the bottom of the beaker in a heterogeneous mixture of a solid and a liquid
Ans : Decantation.
(d) The clean liquid which is poured out after sedimentation is a
Ans : Supernatant liquid.
(e) The technique used to separate the light particles from heavy particles using the flow of wind Ans : Winnowing
