This chapter explores the difference between pure substances and mixtures, explains types of mixtures (homogeneous and heterogeneous), and describes solutions, suspensions, and colloids. You will learn about concentration calculations, the Tyndall effect, and the classification of matter into elements and compounds with their properties.
Introduction
When we buy items like milk, ghee, butter, salt, or juice from the market, we often see the word “pure” written on the packages. For a common person, pure means having no adulteration. However, for a scientist, all these items are actually mixtures of different substances and hence not pure. For example, milk is a mixture of water, fat, proteins, and other components.
When a scientist says something is pure, it means that all the constituent particles of that substance are the same in their chemical nature. A pure substance consists of a single type of particle. In other words, a substance is a pure single form of matter.
As we look around, most of the matter exists as mixtures of two or more pure components. Examples include sea water, minerals, and soil.
2.1 What is a Mixture?
Mixtures are constituted by more than one kind of pure form of matter. We know that dissolved sodium chloride can be separated from water by the physical process of evaporation. However, sodium chloride itself is a pure substance and cannot be separated by physical process into its chemical constituents.
Similarly, sugar is a substance which contains only one kind of pure matter and its composition is the same throughout. Soft drinks and soil are not single pure substances.
Whatever the source of a pure substance may be, it will always have the same characteristic properties.
Definition:
- A mixture contains more than one pure substance.
- Components can be mixed in any proportion.
2.1.1 TYPES OF MIXTURES
Depending upon the nature of the components that form a mixture, we can have different types of mixtures.
Homogeneous Mixtures (Solutions)
When copper sulphate powder is mixed with water:
- A mixture is obtained which has a uniform composition throughout.
- Such mixtures are called homogeneous mixtures or solutions.
- Examples: (i) Salt dissolved in water, (ii) Sugar dissolved in water.
Characteristics:
- The intensity of colour can vary depending on the amount of solute added.
- This shows that a homogeneous mixture can have a variable composition.
Heterogeneous Mixtures
When sodium chloride and iron filings, or salt and sulphur, or oil and water are mixed:
- Mixtures are obtained which contain physically distinct parts.
- They have non-uniform compositions.
- Such mixtures are called heterogeneous mixtures.
Examples:
- Mixture of sodium chloride and iron filings
- Mixture of salt and sulphur
- Mixture of oil and water
Questions
Question 1: What is meant by a pure substance?
Solution:
- A pure substance is a material in which all the constituent particles are the same in their chemical nature.
- It consists of a single type of particle.
- A pure substance has uniform and definite composition.
- Its properties are constant throughout.
- Examples: Pure water, pure gold, pure sugar, pure salt.
Question 2: List the points of differences between homogeneous and heterogeneous mixtures.
Solution:
| Homogeneous Mixture | Heterogeneous Mixture |
|---|---|
| Has uniform composition throughout. | Has non-uniform composition. |
| Components are not visible separately. | Components are physically distinct and visible. |
| Properties are the same throughout. | Properties are different in different parts. |
| Examples: Salt solution, sugar solution, air. | Examples: Sand and salt mixture, oil and water, soil. |
2.2 What is a Solution?
A solution is a homogeneous mixture of two or more substances. You come across various types of solutions in daily life. Lemonade, soda water, etc., are all examples of solutions.
Usually we think of a solution as a liquid that contains either a solid, liquid, or gas dissolved in it. But we can also have:
- Solid solutions (alloys)
- Gaseous solutions (air)
In a solution, there is homogeneity at the particle level. For example, lemonade tastes the same throughout. This shows that particles of sugar or salt are evenly distributed in the solution.
Alloys
Alloys are mixtures of two or more metals or a metal and a non-metal. They cannot be separated into their components by physical methods. However, an alloy is still considered a mixture because:
- It shows the properties of its constituents.
- It can have variable composition.
Example: Brass is a mixture of approximately 30% zinc and 70% copper.
Components of a Solution
A solution has a solvent and a solute as its components.
Solvent:
- The component of the solution that dissolves the other component in it.
- Usually the component present in larger amount.
Solute:
- The component of the solution that is dissolved in the solvent.
- Usually present in lesser quantity.
Examples of Solutions
(i) Sugar in Water:
- Type: Solid in liquid solution.
- Solute: Sugar (solid)
- Solvent: Water (liquid)
(ii) Tincture of Iodine:
- Type: Solid in liquid solution.
- Solute: Iodine (solid)
- Solvent: Alcohol (liquid)
(iii) Aerated Drinks (Soda Water):
- Type: Gas in liquid solution.
- Solute: Carbon dioxide (gas)
- Solvent: Water (liquid)
(iv) Air:
- Type: Gas in gas mixture (homogeneous).
- Main constituents: Oxygen (21%) and Nitrogen (78%).
- Other gases are present in very small quantities.
Properties of a Solution
- A solution is a homogeneous mixture.
- The particles of a solution are smaller than 1 nm (10⁻⁹ metre) in diameter, so they cannot be seen by naked eyes.
- Because of very small particle size, they do not scatter a beam of light passing through the solution. So, the path of light is not visible in a solution.
- The solute particles cannot be separated from the mixture by the process of filtration.
- The solute particles do not settle down when left undisturbed. That is, a solution is stable.
2.2.1 CONCENTRATION OF A SOLUTION
In a solution, the relative proportion of the solute and solvent can be varied. Depending upon the amount of solute present in a solution, it can be called:
- Dilute solution
- Concentrated solution
- Saturated solution
Note: Dilute and concentrated are comparative terms.
Saturated Solution
When salt or sugar is continuously added to water with stirring at a particular temperature:
- After a certain point, no more solute can be dissolved.
- The solution that has dissolved as much solute as it is capable of dissolving at a given temperature is called a saturated solution.
Solubility:
- The amount of the solute present in the saturated solution at a particular temperature is called its solubility.
Unsaturated Solution
If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution.
Effect of Temperature on Solubility
Different substances in a given solvent have different solubilities at the same temperature. When a saturated solution at a certain temperature is cooled slowly, the solubility usually decreases and excess solute may crystallize out.
Methods of Expressing Concentration
There are various ways of expressing the concentration of a solution. Here we will learn three methods:
(i) Mass by mass percentage of a solution:
Mass by mass percentage = (Mass of solute / Mass of solution) × 100
(ii) Mass by volume percentage of a solution:
Mass by volume percentage = (Mass of solute / Volume of solution) × 100
(iii) Volume by volume percentage of a solution:
Volume by volume percentage = (Volume of solute / Volume of solution) × 100
Example 2.1
Question: A solution contains 40 g of common salt in 320 g of water. Calculate the concentration in terms of mass by mass percentage of the solution.
Solution:
- Mass of solute (salt) = 40 g
- Mass of solvent (water) = 320 g
- Mass of solution = Mass of solute + Mass of solvent
- Mass of solution = 40 g + 320 g = 360 g
Mass percentage of solution = (Mass of solute / Mass of solution) × 100
= (40 / 360) × 100
= 11.1%
Answer: The concentration is 11.1%.
2.2.2 WHAT IS A SUSPENSION?
Non-homogeneous systems, in which solids are dispersed in liquids, are called suspensions.
A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium.
Particles of a suspension are visible to the naked eye.
Properties of a Suspension
- Suspension is a heterogeneous mixture.
- The particles of a suspension can be seen by the naked eye.
- The particles of a suspension scatter a beam of light passing through it and make its path visible.
- The solute particles settle down when a suspension is left undisturbed. That is, a suspension is unstable.
- They can be separated from the mixture by the process of filtration.
- When the particles settle down, the suspension breaks and it does not scatter light anymore.
2.2.3 WHAT IS A COLLOIDAL SOLUTION?
A mixture where particles are uniformly spread throughout the solution is called a colloid or a colloidal solution.
The particles of a colloid are uniformly spread throughout the solution. Due to the relatively smaller size of particles (as compared to suspension), the mixture appears to be homogeneous. But actually, a colloidal solution is a heterogeneous mixture.
Example: Milk
Because of the small size of colloidal particles, we cannot see them with naked eyes. But these particles can easily scatter a beam of visible light.
Tyndall Effect
The scattering of a beam of light is called the Tyndall effect (named after the scientist who discovered this effect).
When does Tyndall effect occur?
- When a fine beam of light enters a room through a small hole, the path of light becomes visible due to scattering of light by dust and smoke particles in the air.
- When sunlight passes through the canopy of a dense forest, mist (tiny water droplets) acts as particles of colloid dispersed in air.
- When light passes through milk or a colloid solution.
Note: Solutions do not show Tyndall effect because particles are too small to scatter light.
Properties of a Colloid
- A colloid is a heterogeneous mixture.
- The size of particles of a colloid is too small to be individually seen with naked eyes.
- Colloids are big enough to scatter a beam of light passing through it and make its path visible (Tyndall effect).
- They do not settle down when left undisturbed. That is, a colloid is quite stable.
- They cannot be separated from the mixture by the process of filtration.
- A special technique of separation known as centrifugation can be used to separate the colloidal particles.
Components of a Colloidal Solution
The components of a colloidal solution are:
(i) Dispersed Phase:
- The solute-like component or the dispersed particles in a colloid.
(ii) Dispersion Medium:
- The component in which the dispersed phase is suspended.
Classification of Colloids
Colloids are classified according to the state (solid, liquid, or gas) of the dispersing medium and the dispersed phase.
| Dispersed Phase | Dispersing Medium | Type | Example |
|---|---|---|---|
| Liquid | Gas | Aerosol | Fog, clouds, mist |
| Solid | Gas | Aerosol | Smoke, automobile exhaust |
| Gas | Liquid | Foam | Shaving cream |
| Liquid | Liquid | Emulsion | Milk, face cream |
| Solid | Liquid | Sol | Milk of magnesia, mud |
| Gas | Solid | Foam | Foam rubber, sponge, pumice |
| Liquid | Solid | Gel | Jelly, cheese, butter |
| Solid | Solid | Solid Sol | Coloured gemstone, milky glass |
Questions
Question 1: Differentiate between homogeneous and heterogeneous mixtures with examples.
Solution:
| Homogeneous Mixture | Heterogeneous Mixture |
|---|---|
| Has uniform composition throughout. | Has non-uniform composition. |
| Components are not separately visible. | Components are physically distinct and can be seen. |
| Properties are same in all parts. | Properties vary in different parts. |
| Examples: Salt solution, sugar solution, air, vinegar. | Examples: Sand and salt mixture, oil and water, soil, wood. |
Question 2: How are sol, solution and suspension different from each other?
Solution:
| Property | Solution | Suspension | Sol (Colloid) |
|---|---|---|---|
| Type | Homogeneous mixture | Heterogeneous mixture | Heterogeneous mixture (appears homogeneous) |
| Particle Size | Less than 1 nm | Larger than 1000 nm | Between 1 nm to 1000 nm |
| Visibility | Not visible | Visible to naked eye | Not visible to naked eye |
| Tyndall Effect | Does not show | Shows (until particles settle) | Shows |
| Stability | Stable | Unstable (particles settle) | Stable |
| Filtration | Cannot be separated | Can be separated | Cannot be separated by filtration |
| Examples | Salt in water, sugar in water | Chalk in water, muddy water | Milk, blood, fog |
Question 3: To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water at 293 K. Find its concentration at this temperature.
Solution:
- Mass of solute (sodium chloride) = 36 g
- Mass of solvent (water) = 100 g
- Mass of solution = Mass of solute + Mass of solvent
- Mass of solution = 36 g + 100 g = 136 g
Concentration (mass by mass %) = (Mass of solute / Mass of solution) × 100
= (36 / 136) × 100
= 26.47%
Answer: The concentration is 26.47%.
2.3 Physical and Chemical Changes
In the previous chapter, we have learnt about a few physical properties of matter. The properties that can be observed and specified like colour, hardness, rigidity, fluidity, density, melting point, boiling point, etc., are the physical properties.
Physical Changes
The interconversion of states is a physical change because these changes occur without a change in composition and no change in the chemical nature of the substance.
Although ice, water, and water vapour all look different and display different physical properties, they are chemically the same.
Chemical Changes
Both water and cooking oil are liquids but their chemical characteristics are different. They differ in odour and inflammability. We know that oil burns in air whereas water extinguishes fire. It is this chemical property of oil that makes it different from water.
Burning is a chemical change. During this process, one substance reacts with another to undergo a change in chemical composition.
Chemical change brings change in the chemical properties of matter and we get new substances. A chemical change is also called a chemical reaction.
Example: Burning of a Candle
During burning of a candle, both physical and chemical changes take place:
Physical Changes:
- Melting of wax (solid wax → liquid wax)
- Evaporation of liquid wax
Chemical Changes:
- Burning of wax vapour in air
- Formation of carbon dioxide and water vapour
- Release of heat and light
Questions
Question 1: Classify the following as chemical or physical changes:
- Cutting of trees
- Melting of butter in a pan
- Rusting of almirah
- Boiling of water to form steam
- Passing of electric current through water and the water breaking down into hydrogen and oxygen gases
- Dissolving common salt in water
- Making a fruit salad with raw fruits
- Burning of paper and wood
Solution:
Physical Changes:
- Cutting of trees (No change in chemical composition)
- Melting of butter in a pan (Change of state only)
- Boiling of water to form steam (Change of state only)
- Dissolving common salt in water (No new substance formed)
- Making a fruit salad with raw fruits (No chemical change)
Chemical Changes:
- Rusting of almirah (Iron reacts with oxygen and moisture to form rust)
- Passing electric current through water (Water breaks down into hydrogen and oxygen – electrolysis)
- Burning of paper and wood (New substances formed – ash, carbon dioxide, water vapour)
Question 2: Try segregating the things around you as pure substances or mixtures.
Solution:
Pure Substances:
- Distilled water
- Pure gold
- Pure iron
- Pure sugar
- Pure salt
- Diamond
- Copper wire
Mixtures:
- Tap water
- Milk
- Air
- Soil
- Wood
- Juice
- Tea
- Blood
2.4 What are the Types of Pure Substances?
On the basis of their chemical composition, substances can be classified either as elements or compounds.
2.4.1 ELEMENTS
Robert Boyle was the first scientist to use the term element in 1661.
Antoine Laurent Lavoisier (1743–94), a French chemist, was the first to establish an experimentally useful definition of an element.
Definition:
- He defined an element as a basic form of matter that cannot be broken down into simpler substances by chemical reactions.
Classification of Elements
Elements can be normally divided into:
- Metals
- Non-metals
- Metalloids
Metals
Metals usually show some or all of the following properties:
- They have a lustre (shine).
- They have silvery-grey or golden-yellow colour.
- They conduct heat and electricity.
- They are ductile (can be drawn into wires).
- They are malleable (can be hammered into thin sheets).
- They are sonorous (make a ringing sound when hit).
Examples of Metals:
- Gold
- Silver
- Copper
- Iron
- Sodium
- Potassium
Special Note: Mercury is the only metal that is liquid at room temperature.
Non-Metals
Non-metals usually show some or all of the following properties:
- They display a variety of colours.
- They are poor conductors of heat and electricity.
- They are not lustrous, sonorous, or malleable.
Examples of Non-Metals:
- Hydrogen
- Oxygen
- Iodine
- Carbon (coal, coke)
- Bromine
- Chlorine
Metalloids
Some elements have intermediate properties between those of metals and non-metals. They are called metalloids.
Examples of Metalloids:
- Boron
- Silicon
- Germanium
Important Facts About Elements
- The number of elements known at present are more than 100.
- Ninety-two elements are naturally occurring.
- The rest are man-made.
- Majority of the elements are solid.
- Eleven elements are in gaseous state at room temperature.
- Two elements are liquid at room temperature: Mercury and Bromine.
- Elements gallium and cesium become liquid at a temperature slightly above room temperature (303 K).
2.4.2 COMPOUNDS
A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.
Question: What do we get when two or more elements are combined?
Experiment: Iron and Sulphur
Group I:
- Mix and crush iron filings and sulphur powder.
- This is a physical change.
- Result: A mixture is obtained.
Group II:
- Mix and crush iron filings and sulphur powder.
- Heat this mixture strongly till red hot.
- This is a chemical change.
- Result: A compound (iron sulphide) is formed.
Observations
Testing for Magnetism:
- Group I: Iron filings are still attracted to magnet (mixture retains properties of iron).
- Group II: The product is not attracted to magnet (new compound has different properties).
Adding Carbon Disulphide:
- Group I: Sulphur dissolves in carbon disulphide.
- Group II: The compound does not dissolve.
Adding Dilute Acid:
- Group I: Hydrogen gas is produced (colourless, odourless, combustible).
- Group II: Hydrogen sulphide gas is produced (colourless, smells like rotten eggs).
Conclusions
Group I (Mixture):
- The material obtained is a mixture of two elements — iron and sulphur.
- The properties of the mixture are the same as that of its constituents.
- Components can be separated by physical methods.
Group II (Compound):
- The material obtained is a compound (iron sulphide).
- On heating the two elements strongly, we get a compound which has totally different properties compared to the combining elements.
- The composition of a compound is the same throughout.
- The texture and colour of the compound are the same throughout.
- Components can be separated only by chemical or electrochemical reactions.
Differences Between Mixtures and Compounds
| Mixtures | Compounds |
|---|---|
| Elements or compounds just mix together to form a mixture and no new compound is formed. | Elements react to form new compounds. |
| A mixture has a variable composition. | The composition of each new substance is always fixed. |
| A mixture shows the properties of the constituent substances. | The new substance has totally different properties. |
| The constituents can be separated fairly easily by physical methods. | The constituents can be separated only by chemical or electrochemical reactions. |
Classification of Matter – Graphical Organizer
MATTER
(Solid, Liquid, or Gas)
|
___________________________
| |
PURE SUBSTANCE MIXTURES
| (No Fixed Composition)
_____________ |
| | ___________________
ELEMENTS COMPOUNDS | |
| | HOMOGENEOUS HETEROGENEOUS
| | | |
Cannot be Have fixed Uniform Non-uniform
broken down composition composition composition
to simpler
substances Can be broken
down into
elements by
chemical or
electrochemical
reactions
Examples: Examples: Examples: Examples:
Copper, Water, Sugar in water, Sand and salt,
Oxygen, Methane, Salt in water, Sugar and salt,
Iron, Sugar, Sulphur in Water in oil
Hydrogen, Salt carbon disulphide,
Mercury Water in alcohol
Exercises
Question 1: Which separation technique will you apply for the separation of the following?
(a) Sodium chloride from its solution in water
- Solution: Evaporation – Heat the solution to evaporate water, leaving behind solid sodium chloride.
(b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride
- Solution: Sublimation – Heat the mixture. Ammonium chloride will sublime (change directly to gas), leaving sodium chloride behind.
(c) Small pieces of metal in the engine oil of a car
- Solution: Filtration – Pour the oil through a filter paper. Metal pieces will remain on the filter paper.
(d) Different pigments from an extract of flower petals
- Solution: Chromatography – Use paper or column chromatography to separate different coloured pigments.
(e) Butter from curd
- Solution: Centrifugation or Churning – Churn the curd to separate butter from buttermilk.
(f) Oil from water
- Solution: Using separating funnel – Oil floats on water. Use a separating funnel to separate the two immiscible liquids.
(g) Tea leaves from tea
- Solution: Filtration using a strainer – Pour tea through a strainer to separate tea leaves.
(h) Iron pins from sand
- Solution: Magnetic separation – Use a magnet to attract and separate iron pins from sand.
(i) Wheat grains from husk
- Solution: Winnowing – Use wind to blow away lighter husk, leaving heavier wheat grains.
(j) Fine mud particles suspended in water
- Solution: Filtration followed by decantation or centrifugation – Allow particles to settle, then pour off clear water (decantation), or use centrifugation for very fine particles.
Question 2: Write the steps you would use for making tea. Use the words: solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.
Solution:
Steps for Making Tea:
- Take water (acts as solvent) in a pan and heat it.
- Add tea leaves to the boiling water. Tea leaves contain soluble compounds that will dissolve.
- Add sugar (acts as solute) to the hot water. Sugar is soluble in water and will dissolve to form a solution.
- Add milk to the mixture. Milk mixes with water.
- Boil the mixture for some time to extract flavour from tea leaves.
- Tea leaves are insoluble in water and remain as solid particles.
- Filter the tea using a strainer to separate tea leaves.
- The liquid that passes through the strainer is the filtrate (the tea solution we drink).
- The tea leaves that remain on the strainer are the residue.
- The tea is now ready to drink.
Question 3: Pragya tested the solubility of three different substances at different temperatures and collected the data as given below (results are given as grams of substance dissolved in 100 grams of water to form a saturated solution).
| Substance Dissolved | Temperature in K | ||||
|---|---|---|---|---|---|
| 283 | 293 | 313 | 333 | 353 | |
| Potassium nitrate | 21 | 32 | 62 | 106 | 167 |
| Sodium chloride | 36 | 36 | 36 | 37 | 37 |
| Potassium chloride | 35 | 35 | 40 | 46 | 54 |
| Ammonium chloride | 24 | 37 | 41 | 55 | 66 |
(a) What mass of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 50 grams of water at 313 K?
Solution:
- At 313 K, solubility of potassium nitrate = 62 g in 100 g of water
- For 50 g of water: (62 / 100) × 50 = 31 g
- Answer: 31 g of potassium nitrate is needed.
(b) Pragya makes a saturated solution of potassium chloride in water at 353 K and leaves the solution to cool at room temperature. What would she observe as the solution cools? Explain.
Solution:
- At 353 K, solubility of potassium chloride = 54 g in 100 g water
- At room temperature (293 K), solubility = 35 g in 100 g water
- As the solution cools, solubility decreases
- The excess potassium chloride (54 – 35 = 19 g) will crystallize out and settle at the bottom
- Observation: Crystals of potassium chloride will appear in the solution as it cools.
(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?
Solution:
- At 293 K:
- Potassium nitrate = 32 g
- Sodium chloride = 36 g
- Potassium chloride = 35 g
- Ammonium chloride = 37 g
- Answer: Ammonium chloride has the highest solubility at 293 K (37 g per 100 g water).
(d) What is the effect of change of temperature on the solubility of a salt?
Solution:
- From the table, we observe that:
- For potassium nitrate, potassium chloride, and ammonium chloride: Solubility increases significantly with increase in temperature.
- For sodium chloride: Solubility remains almost constant with change in temperature (very slight increase).
- General Effect: For most salts, solubility increases with increase in temperature.
- However, the rate of increase varies from salt to salt.
Question 4: Explain the following giving examples:
(a) Saturated solution
Solution:
- A saturated solution is a solution that has dissolved the maximum amount of solute it can hold at a given temperature.
- At this point, no more solute can dissolve in the solution.
- If more solute is added, it will settle at the bottom and not dissolve.
- Example: If 36 g of salt is dissolved in 100 g of water at room temperature, the solution becomes saturated. Adding more salt will not dissolve it.
(b) Pure substance
Solution:
- A pure substance is made up of only one type of particle.
- It has a uniform and definite composition throughout.
- Its properties are constant and do not vary.
- Examples: Pure water (H₂O), pure gold, pure oxygen, pure sugar, pure salt.
Question 5: Classify each of the following as a homogeneous or heterogeneous mixture:
- Soda water, wood, air, soil, vinegar, filtered tea.
Solution:
Homogeneous Mixtures:
- Soda water – Carbon dioxide uniformly dissolved in water
- Air – Uniform mixture of gases
- Vinegar – Acetic acid uniformly dissolved in water
- Filtered tea – Uniform solution after filtration
Heterogeneous Mixtures:
- Wood – Made of different types of cells and tissues
- Soil – Mixture of sand, clay, organic matter (non-uniform)
Question 6: How would you confirm that a colourless liquid given to you is pure water?
Solution:
Test 1: Boiling Point Test
- Heat the liquid and measure its boiling point.
- Pure water boils at exactly 100°C (373 K) at normal atmospheric pressure.
- If the liquid boils at 100°C, it is likely pure water.
Test 2: Freezing Point Test
- Cool the liquid and measure its freezing point.
- Pure water freezes at exactly 0°C (273 K).
- If it freezes at 0°C, it confirms purity.
Test 3: Evaporation Test
- Take a small amount of liquid in a dish.
- Heat it to evaporate completely.
- Pure water will leave no residue after evaporation.
- If any residue remains, the water is not pure.
Test 4: Physical Properties
- Pure water is colourless, odourless, and tasteless.
- Check these properties.
Question 7: Which of the following materials fall in the category of a “pure substance”?
(a) Ice, (b) Milk, (c) Iron, (d) Hydrochloric acid, (e) Calcium oxide, (f) Mercury, (g) Brick, (h) Wood, (i) Air
Solution:
Pure Substances:
- (a) Ice – Pure H₂O in solid state
- (c) Iron – Element (Fe)
- (d) Hydrochloric acid – Compound (HCl dissolved in water forms a compound)
- (e) Calcium oxide – Compound (CaO)
- (f) Mercury – Element (Hg)
Not Pure Substances (Mixtures):
- (b) Milk – Mixture of water, fat, proteins, etc.
- (g) Brick – Mixture of clay, sand, and other materials
- (h) Wood – Mixture of cellulose, lignin, and other compounds
- (i) Air – Mixture of gases (nitrogen, oxygen, etc.)
Question 8: Identify the solutions among the following mixtures:
(a) Soil, (b) Sea water, (c) Air, (d) Coal, (e) Soda water
Solution:
Solutions (Homogeneous mixtures):
- (b) Sea water – Salt dissolved uniformly in water
- (c) Air – Uniform mixture of gases
- (e) Soda water – Carbon dioxide gas dissolved in water
Not Solutions:
- (a) Soil – Heterogeneous mixture
- (d) Coal – Heterogeneous mixture (solid)
Question 9: Which of the following will show “Tyndall effect”?
(a) Salt solution, (b) Milk, (c) Copper sulphate solution, (d) Starch solution
Solution:
Show Tyndall Effect (Colloids):
- (b) Milk – Colloidal solution (emulsion)
- (d) Starch solution – Colloidal solution (sol)
Do Not Show Tyndall Effect (True Solutions):
- (a) Salt solution – True solution (particles too small)
- (c) Copper sulphate solution – True solution (particles too small)
Explanation: Tyndall effect is shown by colloids where particle size is between 1 nm to 1000 nm, large enough to scatter light.
Question 10: Classify the following into elements, compounds and mixtures:
(a) Sodium, (b) Soil, (c) Sugar solution, (d) Silver, (e) Calcium carbonate, (f) Tin, (g) Silicon, (h) Coal, (i) Air, (j) Soap, (k) Methane, (l) Carbon dioxide, (m) Blood
Solution:
Elements:
- (a) Sodium (Na)
- (d) Silver (Ag)
- (f) Tin (Sn)
- (g) Silicon (Si)
Compounds:
- (e) Calcium carbonate (CaCO₃)
- (k) Methane (CH₄)
- (l) Carbon dioxide (CO₂)
Mixtures:
- (b) Soil – Heterogeneous mixture
- (c) Sugar solution – Homogeneous mixture
- (h) Coal – Heterogeneous mixture
- (i) Air – Homogeneous mixture of gases
- (j) Soap – Mixture of compounds
- (m) Blood – Heterogeneous mixture (colloid)
Question 11: Which of the following are chemical changes?
(a) Growth of a plant, (b) Rusting of iron, (c) Mixing of iron filings and sand, (d) Cooking of food, (e) Digestion of food, (f) Freezing of water, (g) Burning of a candle
Solution:
Chemical Changes (New substances formed):
- (a) Growth of a plant – Complex biochemical reactions
- (b) Rusting of iron – Iron reacts with oxygen to form iron oxide
- (d) Cooking of food – Chemical composition changes
- (e) Digestion of food – Food breaks down into simpler substances
- (g) Burning of a candle – Wax reacts with oxygen to form CO₂ and H₂O
Physical Changes (No new substances):
- (c) Mixing of iron filings and sand – Just physical mixing
- (f) Freezing of water – Only state change (liquid to solid)
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