Grade 6 Chapter 5: Substances in the Surroundings – Their States and Properties

Introduction

Everything around us, from the air we breathe to the chair we sit on, is made up of something called matter. Matter exists in different forms or states, and each substance has unique characteristics or properties. In this chapter, we will explore the three main states of matter – solid, liquid, and gas – and understand how substances can change from one state to another. We will also learn about various properties of substances, such as how hard or soft they are, whether light can pass through them, and if they dissolve in water, which helps us to identify and use them effectively.

Key topics covered:

• States of Matter: Solid, Liquid, Gas
• Changes of State: Melting, Freezing, Evaporation, Condensation, Sublimation
• Melting Point and Boiling Point
• Properties of Substances:
  • Hardness and Brittleness
  • Transparency and Opacity
  • Solubility
  • Conductivity (Heat and Electricity)
  • Density

States of Matter

Matter commonly exists in three states: solid, liquid, and gas. These states differ in how their particles are arranged and how much they can move.

1. Solid State

• Fixed shape and volume: Solids have a definite shape and volume. They do not change their shape easily.
• Particles are tightly packed: The particles (atoms or molecules) in a solid are very close together and arranged in a fixed pattern.
• Limited movement: Particles can only vibrate in their fixed positions.
• Examples: Stone, wood, iron, ice, sugar, book.

2. Liquid State

• Fixed volume, no fixed shape: Liquids have a definite volume but take the shape of the container they are in.
• Particles are loosely packed: Particles are close together but are not rigidly fixed. They can move past each other.
• Flow easily: Liquids can flow from one place to another.
• Examples: Water, milk, oil, juice, petrol.

3. Gaseous State

• No fixed shape or volume: Gases have neither a definite shape nor a definite volume. They fill the entire container they are in.
• Particles are very far apart: Particles are widely spaced and move randomly and rapidly in all directions.
• Highly compressible: Gases can be easily compressed.
• Examples: Air, oxygen, nitrogen, steam, LPG.

Changes of State

Substances can change from one state to another by heating or cooling. This change is physical and reversible.

• Melting (Fusion): The process by which a solid changes into a liquid on heating.
  • Melting Point: The specific temperature at which a solid melts into a liquid at atmospheric pressure. For ice, the melting point is $0^\circ \text{C}$.
• Freezing (Solidification): The process by which a liquid changes into a solid on cooling.
  • The freezing point of a substance is the same as its melting point. For water, the freezing point is $0^\circ \text{C}$.
• Evaporation (Vaporization): The process by which a liquid changes into a gas (vapor) on heating. This can happen at any temperature below the boiling point (slow evaporation) or rapidly at the boiling point (boiling).
  • Boiling Point: The specific temperature at which a liquid changes into a gas rapidly, forming bubbles throughout the liquid, at atmospheric pressure. For water, the boiling point is $100^\circ \text{C}$.
• Condensation: The process by which a gas (vapor) changes into a liquid on cooling.
  • Condensation often occurs when water vapor cools down (e.g., formation of dew, clouds).
• Sublimation: The process by which a solid changes directly into a gas (vapor) without passing through the liquid state, on heating.
  • Examples: Naphthalene balls, camphor, dry ice (solid carbon dioxide).

Properties of Substances

Different substances have different characteristics or properties that help us identify and classify them.

1. Hardness and Brittleness

• Hardness: The ability of a substance to resist scratching or indentation. Hard substances are difficult to scratch (e.g., diamond, iron).
• Softness: Substances that can be easily compressed or scratched (e.g., sponge, rubber).
• Brittleness: The property of a substance to break easily into pieces when force is applied (e.g., glass, chalk, clay pot).
• Toughness: The ability of a substance to resist breaking or fracturing (e.g., iron, wood).

2. Transparency, Translucency, and Opacity

These properties describe how light passes through a substance.

• Transparency: Substances through which light can pass completely, allowing us to see clearly through them (e.g., clear glass, pure water, air).
• Translucency: Substances through which light can pass partially, making objects appear blurred or diffused (e.g., frosted glass, butter paper, tracing paper).
• Opacity: Substances through which light cannot pass at all, blocking vision completely (e.g., wood, metal, stone, cardboard).

3. Solubility

The ability of a substance (solute) to dissolve in another substance (solvent) to form a solution.

• Soluble: Substances that dissolve completely in a solvent (e.g., sugar in water, salt in water).
• Insoluble: Substances that do not dissolve in a solvent (e.g., sand in water, oil in water).

Water is often called the universal solvent because it can dissolve a large number of substances.

4. Conductivity (Heat and Electricity)

The ability of a substance to allow heat or electricity to pass through it.

• Good Conductors: Substances that allow heat or electricity to pass through them easily (e.g., metals like copper, iron, aluminum for heat and electricity).
• Bad Conductors (Insulators): Substances that do not allow heat or electricity to pass through them easily (e.g., wood, plastic, rubber for both heat and electricity; air for heat).

5. Density

Density is a measure of how much mass is contained in a given volume of a substance. It determines whether an object will float or sink in a liquid.

Density = $\frac{\text{Mass}}{\text{Volume}}$

• Floating: Objects that are less dense than the liquid they are in will float (e.g., wood in water, ice in water).
• Sinking: Objects that are denser than the liquid they are in will sink (e.g., stone in water, iron nail in water).