Is Matter Around Us Pure : Comprehensive Class 9 Notes for Students | cbse24

 Table of Contents

1. Pure Substance
2. Impure Substance
3. Solution, Suspension and Collide
4. Concentration of a Solution
5. Saturated and Unsaturated solution
6. Physical and chemical Changes
7. Separation of Mixtures

What is a substance?

• Anything that cannot be further divided into simpler particles through physical processes is called a substance. 
• Matter can be classified into two categories: pure substances and mixtures.

[1] PURE SUBSTANCE: ELEMENTS AND COMPOUNDS

A pure substance is made up of only one kind of particles. These particles may be atoms or molecules.

For example, sulfur is a pure substance because it consists only of sulfur atoms, while water is a pure substance because it contains only water molecules. Both elements and compounds are pure substances as they are made up of just one type of particle.

Examples include:

• Elements: Hydrogen, Oxygen, Iron, Gold.
• Compounds: Water, Carbon Dioxide, Sodium Chloride, Sugar.

A pure substance is homogeneous and cannot be separated into other types of matter by physical means. It has a fixed composition, melting point, and boiling point.

[2] IMPURE SUBSTANCE: MIXTURES

A mixture consists of two or more different types of particles (atoms or molecules) and contains two or more pure substances combined together.

Some common mixtures include:

• Salt Solution
• Sugar Solution
• Milk
• Sea Water
• Air
• Sugarcane Juice
• Soft Drinks
• Tea
• Coffee
• Soil

[1.1] ELEMENT:

An element is a substance that cannot be broken down into simpler substances through common chemical methods such as heat, light, or electricity. 

 An element is a substance which is made of only one kind of atom.

Based on their properties, all the elements can be divided into three groups : 

1. Metals,

2. Non-metals, 

3. Metalloids

Metals

A metal is an element that is malleable, ductile, and conducts electricity. Examples are Iron, Copper, and Gold. Most metals are solid, except mercury, which is a liquid.

Properties of Metals:-

1. Metals are Malleable. This means that metals can be beaten into thin sheets with a hammer (without breaking). Gold and silver metals are some of the best malleable metals.

2. Metals are Ductile. This means that metals can be drawn (or stretched) into thin wires. All the metals are not equally ductile. Some are more ductile than the others. Gold and silver are among the best ductile metals.

3:-Metals are excellent conductors of heat and electricity, allowing them to pass through easily. Silver is the best conductor of heat, followed by copper and aluminum.

4:-Lustrous:-Metals are lustrous and can be polished to a shiny surface. For example, gold, silver, and copper are known for their shine. This property is called metallic lustre.

5:-Metals are Sonorous. This means that metals make a ringing sound when we strike them

Non-Metals

A non-metal is an element that is neither malleable nor ductile and does not conduct electricity. Examples include Carbon, Sulphur, Hydrogen, and Oxygen. Non-metals are typically solids or gases, with bromine being the only liquid non-metal at room temperature. Diamond and Graphite are non-metal forms of carbon.

Properties of Non-Metals 

1:-Non-metals are not malleable and are brittle, meaning they cannot be hammered into thin sheets and instead break into small pieces when struck. Examples include sulfur and phosphorus.

2. Non-Metals are Not Ductile. This means that non-metals cannot be drawn into wires.

3:-Non-Metals are Bad Conductors of Heat and Electricity. This means that non-metals do not allow heat and electricity to pass through them. For example, sulphur and phosphorus

4:-Non-Metals are Not Lustrous (Not Shiny). They are Dull in Appearance. Non-metals do not have lustre (chamak) which means that non-metals do not have a shining surface

5. Non-metals are Generally Soft (except diamond which is extremely hard non-metal). Most of the solid non-metals are quite soft.

Metalloids

Elements that exhibit properties of both metals and non-metals are known as metalloids. Their characteristics are intermediate between metals and non-metals. Metalloids are also referred to as semi-metals. Key examples include Boron (B), Silicon (Si), and Germanium (Ge).

Quick Facts:

• There are 118 known elements.
• 92 are naturally occurring.
• 26 are man-made.
• Most elements are solids at room temperature.
• 11 elements are gases at room temperature.
• 2 elements are liquids at room temperature: bromine and mercury.
• 2 more elements, calcium and gallium, become liquid slightly above room temperature.

[2.2]   MIXTURES

A mixture is a substance with two or more elements or compounds that are not chemically combined. For example, the air is a mixture of oxygen, nitrogen, argon, carbon dioxide, and water vapour.

Examples of mixtures include air, gunpowder, brass, salt solution, sugar solution, milk, seawater, ink, kerosene, petrol, petroleum, lime water, paint, glass, coal, soil, wood, blood, starch solution, soap solution, iron and sulfur mixture, dyes, alcohol and water, petrol and water, chalk-water mixture, soda water, soft drinks, lemonade, vinegar, muddy river water, flour in water, milk of magnesia, butter, cheese, face cream, shaving cream, hairspray, smoke, fog, and mist.

Types of Mixtures Mixtures are of two types: 

Homogenous Mixture:-.

Homogeneous mixtures are those in which the substances are completely mixed and indistinguishable from one another.

Homogeneous mixtures, or solutions, include examples like sugar solution, salt solution, copper sulfate solution, seawater, alcohol and water mixture, petrol and oil mixture, soda water, soft drinks, lemonade, vinegar, brass, air, kerosene, and petrol. Note that kerosene and petrol are mixtures of various hydrocarbons, not single substances.

Heterogeneous Mixture:-

Those mixtures in which the substances remain separate and one substance is spread throughout the other substance as small particles, droplets or bubbles, are called heterogeneous mixtures

 

 The suspensions of solids in liquids are also heterogeneous For example, a suspension of chalk in water is a heterogeneous mixture.

[1.2] COMPOUND

A compound is a substance formed from two or more elements chemically combined in a fixed proportion by mass. For example, water (H₂O) consists of hydrogen and oxygen in a 1:8 mass ratio. Other examples include common salt (NaCl), ammonium chloride (NH₄Cl), silicon dioxide (SiO₂), and marble (CaCO₃).

Difference between Mixture and Compound

Though a compound is always homogeneous, a mixture may be heterogeneous or homogeneous.

What is an alloy?

An alloy is a mixture of different metals or nonmetals and metals that cannot be separated from each other using physical methods.

For Example
Brass – Copper with up to 50% zinc
Bronze – Copper with up to 12% tin

 

[3] SOLUTIONS, SUSPENSIONS AND COLLOIDS

(a) Solution

The substance dissolved in a liquid to make a solution is called the **solute**, while the liquid in which the solute is dissolved is known as the **solvent**. For example, salt is the solute in a salt solution and water is the solvent.

Solution = Solute + Solvent

Properties of a Solution:

• A solution is a homogeneous mixture.
• Solute particles are extremely small, less than 1 nm in diameter.
• Particles cannot be seen with a microscope.
• Particles pass through filter paper; solutions cannot be separated by filtration.
• Solutions are stable; solute particles do not separate out over time.
• A true solution does not scatter light due to its tiny particles.

Types of Solutions:

• Dilute: Contains a small amount of solute relative to the solvent. Example: 1 g of salt in 500 ml of water.
• Unsaturated: Can still dissolve more solute; not yet at saturation.
• Concentrated: Contains a large amount of solute.
• Saturated: Has dissolved the maximum amount of solute possible; no more solute can be added.

(b) Suspension

Substances that dissolve in water create solutions, whereas those that don't form suspensions. 

A suspension is a heterogeneous mixture where solid particles are spread throughout a liquid without dissolving. Examples include chalk in water, muddy water, milk of magnesia, sand in water, and flour in water.

Properties of a Suspension:

• A suspension is a heterogeneous mixture.
• Its particles are larger than 100 nm in diameter.
• The particles are visible to the naked eye.
• The particles do not pass through filter paper and can be separated by filtration.
• Suspensions are unstable; particles settle over time.
• Suspensions scatter light due to their large particles.

(c) Colloid 

A colloid is a type of solution where the size of the solute particles is intermediate between those in true solutions and suspensions.

Examples of colloids (or colloidal solutions) include soap solutions, starch solutions, milk, ink, blood, jelly, and synthetic detergent solutions.

Properties of Colloids:

1. Colloids appear homogeneous but are actually heterogeneous.
2. Particle size is between 1 nm and 100 nm, larger than in true solutions but smaller than in suspensions.
3. Particles are typically not visible even under a microscope.
4. Colloids pass through filter paper and cannot be separated by filtration.
5. We cannot separate colloidal particles through filtration. Instead, we use centrifugation to separate them.
6. Colloids scatter light, making a beam of light visible through them.

Classification of Colloids

Colloids can involve solids, liquids, and gases and are classified based on the physical state of the dispersed phase (solute) and the dispersion medium (solvent). The main types of colloids are:

• Sol
• Solid sol
• Aerosol
• Emulsion
• Foam
• Solid foam
• Gel

[4] CONCENTRATION OF A SOLUTION

The concentration of a solution is the amount of solute present in a given quantity of the solution.

(a) The concentration of a solution 

 The formula for calculating the weight percentage (w/w) concentration of a solution is:

$$\text{Concentration (\% w/w)}=\left(\frac{\text{Mass of solute}}{\text{Mass of solution}}\right)\times 100$$

Here’s how you use this formula:

1. Mass of solute: The mass of the solute (in grams) that is dissolved in the solution.
2. Mass of solution: The total mass of the solution (solvent + solute).

Example Calculation:

If you have a solution where 10 grams of common salt (solute) are dissolved in 90 grams of water (solvent), the total mass of the solution is:

Mass of solution=Mass of solute+Mass of solvent=10 grams+90 grams=100 grams

Now, calculate the concentration:

$$\text{Concentration (\% w/w)} = \left( \frac{10 \text{ grams}}{100 \text{ grams}} \right) \times 100 = 10\%$$

So, the concentration of the solution is 10% (w/w).

(b) The Case of a Liquid Solute Dissolved in a Liquid Solvent

When a liquid solute is dissolved in a liquid solvent, the concentration is often expressed as the volume percentage of the solute. This is calculated as:

$$\text{Volume \% of solute}=\left(\frac{\text{Volume of solute}}{\text{Total volume of solution}}\right)\times 100\mathrm{<br>}$$

For example, if you have 30 mL of solute in a 150 mL solution, the volume percentage is:

$$\text{Volume \% of solute} = \left( \frac{30 \, \text{mL}}{150 \, \text{mL}} \right) \times 100 = 20\%$$

This means that 20% of the total volume of the solution is made up of the solute.

[5] SATURATED AND UNSATURATED SOLUTIONS

1. Unsaturated Solution: Contains less solute than the maximum amount that can dissolve at a given temperature. More solute can still be added without changing the temperature.

2. Saturated Solution: Contains the maximum amount of solute that can dissolve at a given temperature. No more solutes can be dissolved without altering the temperature.

Example: 

• Unsaturated: A saltwater solution where more salt can still be dissolved.
• Saturated: A saltwater solution where additional salt remains undissolved at that temperature.

[6] Solubility 

Solubility is a measure of how much solute can dissolve in a solvent at a specific temperature and pressure. It indicates the maximum concentration of the solute that can be achieved in the solution.

Definition:

• Solubility refers to the maximum amount of solute that can dissolve in a given quantity of solvent to form a stable solution at a specified temperature and pressure.

Units:

• Typically expressed in grams of solute per 100 grams of solvent (g/100 g), or in moles per liter (Molarity, M).

Factors Affecting Solubility:

• Temperature: For most solids, solubility increases with temperature. For gases, solubility usually decreases with increasing temperature.
• Pressure: Affects the solubility of gases; higher pressure increases gas solubility in liquids.
• Nature of Solute and Solvent: "Like dissolves like" – polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes.

Example:

Salt in Water: At room temperature, the solubility of common salt (sodium chloride) in water is about 36 grams per 100 grams of water. This means you can dissolve up to 36 grams of salt in 100 grams of water before reaching a saturated solution

.
Example:-12 grams of potassium sulphate dissolves in 75 grams of water at 60°C. What is its solubility in water at that temperature?

To calculate the solubility of potassium sulfate in water at 60°C, you can use the following formula:

Solubility (g/100 g of water)=(Mass of solute mass of solvent)×100

Given:

• Mass of potassium sulfate (solute) = 12 grams
• Mass of water (solvent) = 75 grams

Calculation:

$$\text{Solubility}=\left(\frac{12\text{ grams}}{75\text{ grams}}\right)\times 100\mathrm{<br>}\mathrm{<br>}$$$$\text{Solubility} = \left( \frac{12}{75} \right) \times 100$$
$$\text{Solubility} = 0.16 \times 100$$
$$\text{Solubility}=16\text{ grams per 100 grams of water }\text{<br>}$$

So, the solubility of potassium sulfate in water at 60°C is 16 grams per 100 grams of water.

[7] PHYSICAL AND CHEMICAL CHANGES

Physical Change

Definition: A physical change affects only the physical properties of a substance, such as its shape, size, or state, without altering its chemical composition.

Characteristics:

• Reversible: Often, physical changes can be reversed (e.g., freezing and melting).
• No New Substance: The substance remains the same chemically before and after the change.

Examples:

• Melting: Ice melting into water.
• Dissolving: Sugar dissolves in water.
• Cutting: Cutting paper into smaller pieces.
• Phase Changes: Boiling water to produce steam.

Chemical Change

Definition: A chemical change results in the formation of one or more new substances with different chemical properties. The chemical composition of the original substance is altered.

Characteristics:

• Irreversible: Often, chemical changes cannot be easily reversed (e.g., burning wood).
• New Substances: New substances with different chemical properties are formed.

Indicators:

• Colour Change: Color change, such as leaves turning brown.
• Temperature Change: Heat is released or absorbed, such as in combustion.
• Gas Production: Formation of gas bubbles, such as when vinegar reacts with baking soda.
• Precipitate Formation: Formation of a solid from a solution, such as when mixing two clear solutions to form a solid.

Examples:

• Burning: Burning paper to form ash and gases.
• Rusting: Iron rusting to form iron oxide.
• Fermentation: Yeast converting sugar into alcohol and carbon dioxide.
• Cooking: Baking a cake, which involves chemical reactions to create a new substance.

[8] SEPARATION OF MIXTURES

Separation of mixtures involves various techniques to isolate individual components from a mixture based on their physical or chemical properties. Here’s a summary of common methods used for separating mixtures:

Physical Methods for Heterogeneous Mixtures:

• Filtration: Separates solids from liquids or gases using a filter.
• Hand-picking: Manually selecting and separating different components.
• Sieving: Separates particles based on size using a sieve.

Additional Separation Techniques:

• Evaporation: Removes a liquid to leave behind a solid.
• Centrifugation: Uses centrifugal force to separate components based on density.
• Sublimation: Separates substances that change from solid to gas without becoming liquid.
• Chromatography: Separates components based on their movement through a medium.
• Distillation: Separates liquids based on differences in boiling points

1. Filtration

Principle: Separates solids from liquids or gases based on particle size.

How It Works: A mixture is poured through a filter paper or mesh that allows the liquid or gas to pass through while retaining the solid particles.

Example: Separating sand from water.

2. Distillation

Principle: Separates substances based on differences in boiling points.

How It Works: Distillation is the process of heating a liquid to form vapour and then cooling the vapour to get back liquid.

Example: Purifying water from saltwater.

3. Evaporation

Principle: Separates a soluble solid from a liquid by evaporating the liquid.

How It Works: The liquid is heated until it evaporates, leaving the solid residue behind.

Example: Obtaining salt from seawater.

4. Centrifugation

Principle: Separates components based on their densities using centrifugal force.

How It Works: The mixture is spun rapidly in a centrifuge, causing denser particles to move to the bottom of the container, forming a sediment.

Example: Separating blood components (like plasma and red blood cells).

5. Chromatography

Chromatography is a technique of separating two (or more) dissolved solids which are present in a solution in very small quantities. 

Principle: Separates components based on their different affinities to a stationary phase and a mobile phase.

How It Works:. This separation is based on the fact that though two (or more) substances are soluble in the same solvent their solubilities may be different.

Example: Separating pigments in ink or dyes.

6. Magnetism

Principle: Separates magnetic materials from non-magnetic ones.

How It Works: A magnet is used to attract magnetic materials, leaving non-magnetic materials behind.

Example: Separating iron filings from a mixture of sand and iron.

7. Sublimation

Principle: the changing of a solid directly into vapours on heating, and of vapours into a solid on cooling is called sublimation.

How It Works: The mixture is heated, and the sublimating substance turns into a gas and can be collected, leaving other components behind.

Example: Separating iodine crystals from a mixture with non-sublimating substances.

8. Decantation

Principle: Separates liquids with different densities or a liquid from a solid.

How It Works: The mixture is allowed to settle, and the top liquid layer is carefully poured off.

Example: Separating oil from water.

9. Sieving

Principle: Separates particles based on size.

How It Works: A sieve with specific mesh sizes is used to separate particles of different sizes.

Example: Separating gravel from sand.

These methods can be used individually or in combination, depending on the nature of the mixture and the properties of its components.

CBSE Class 9 Science Chapter 2: Is Matter Around Us Pure?

Overview:

• Content: The chapter explores pure and impure substances, mixtures, solvents, heterogeneous and homogeneous solutions, and colloids.
• Importance: Essential for understanding everyday and advanced scientific concepts.

Study Tips:

1. Read Thoroughly: Understand each term in the NCERT chapter.
2. Make Notes: Not down key points.
3. Practice: Solve NCERT and NCERT Exemplar problems.
4. Use Resources: Refer to NCERT Solutions and online revision notes.