Comprehensive Notes on Matter Around Us

Pure Substances and Mixtures

What is a Pure Substance?

• In everyday language, "pure" means unadulterated.
• In science, a pure substance consists of a single type of particle; all constituent particles are the same in chemical nature.
• A pure substance is a single form of matter.
• Most matter around us exists as mixtures of two or more pure components (e.g., seawater, minerals, soil).

What is a Mixture?

• Mixtures are constituted by more than one kind of pure form of matter.
• Components of a mixture can be separated by physical processes (e.g., evaporation of salt from water).
• A pure substance (e.g., sodium chloride, sugar) cannot be separated into its chemical constituents by physical processes.
• A mixture contains more than one pure substance, and the source of a pure substance does not affect its characteristic properties.

Types of Mixtures

• Mixtures can be homogeneous or heterogeneous, depending on the nature of their components.
  • Homogeneous Mixtures: Have a uniform composition throughout.
    • Examples: salt dissolved in water, sugar dissolved in water.
    • Homogeneous mixtures can have variable compositions; the intensity of color can differ even with the same components.
  • Heterogeneous Mixtures: Contain physically distinct parts and have non-uniform compositions.
    • Examples: mixtures of sodium chloride and iron filings, salt and sulphur, oil and water.

Solutions, Suspensions, and Colloidal Solutions

What is a Solution?

• A solution is a homogeneous mixture of two or more substances.
• Examples: lemonade, soda water.
• Solutions can be solid (alloys), liquid, or gaseous (air).
• Solutions exhibit homogeneity at the particle level (e.g., lemonade tastes the same throughout).
• Alloys are mixtures of two or more metals or a metal and a non-metal and cannot be separated into their components by physical methods.
  • An alloy is considered a mixture because it shows the properties of its constituents and can have variable composition.
  • Example: Brass is approximately 30% zinc and 70% copper.
• A solution has a solvent and a solute.
  • Solvent: The component that dissolves the other component (usually present in larger amount).
  • Solute: The component that is dissolved in the solvent (usually present in lesser quantity).
• Examples:
  • Sugar in water: sugar is the solute, water is the solvent (solid in liquid solution).
  • Tincture of iodine: iodine is the solute, alcohol is the solvent (solid in liquid solution).
  • Aerated drinks: carbon dioxide is the solute, water is the solvent (gas in liquid solution).
  • Air: oxygen (21%) and nitrogen (78%) are the main constituents (gas in gas solution).
• Properties of a solution:
  • Homogeneous mixture.
  • Particles are smaller than $1 \text{ nm } (10^{-9} \text{ metre})$ in diameter.
  • Particles cannot be seen with the naked eye.
  • Does not scatter a beam of light passing through the solution; the path of light is not visible.
  • Solute particles cannot be separated by filtration.
  • Solute particles do not settle down; a solution is stable.

Concentration of a Solution

• The relative proportion of solute and solvent can be varied in a solution.
• Solutions can be dilute, concentrated, or saturated, depending on the amount of solute present.
  • Saturated solution: A solution that has dissolved as much solute as it is capable of dissolving at a particular temperature.
  • Solubility: The amount of solute present in the saturated solution at a given temperature.
  • Unsaturated solution: A solution in which the amount of solute is less than the saturation level.
• Different substances in a given solvent have different solubilities at the same temperature.
• The concentration of a solution is the amount (mass or volume) of solute present in a given amount (mass or volume) of solution.
• Ways of expressing concentration:
  • Mass by mass percentage: $\frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100$
  • Mass by volume percentage: $\frac{\text{Mass of solute}}{\text{Volume of solution}} \times 100$
  • Volume by volume percentage: $\frac{\text{Volume of solute}}{\text{Volume of solution}} \times 100$
• Example Calculation:
  • A solution contains $40 \text{ g}$ of common salt in $320 \text{ g}$ of water.
  • Mass of solute (salt) = $40 \text{ g}$
  • Mass of solvent (water) = $320 \text{ g}$
  • Mass of solution = $40 \text{ g} + 320 \text{ g} = 360 \text{ g}$
  • Mass percentage of solution = $\frac{40}{360} \times 100 = 11.1\%$

What is a Suspension?

• Non-homogeneous systems in which solids are dispersed in liquids.
• 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:
  • Heterogeneous mixture.
  • Particles can be seen with the naked eye.
  • Scatters a beam of light passing through it and makes its path visible.
  • The solute particles settle down when left undisturbed (unstable).
  • Can be separated by filtration.
  • When particles settle, the suspension breaks and does not scatter light anymore.

What is a Colloidal Solution?

• A mixture in which the particles are uniformly spread throughout the solution.
• The mixture appears to be homogeneous but is actually heterogeneous.
• Particles are relatively small but bigger than solution particles.
• Colloidal particles cannot be seen with the naked eye but can scatter a beam of visible light (Tyndall effect).
• Tyndall Effect: Scattering of a beam of light by colloidal particles.
• Tyndall effect can be observed when a fine beam of light enters a room through a small hole due to scattering of light by dust and smoke particles.
• 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.
  • 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. But, a special technique of separation known as centrifugation can be used to separate the colloidal particles.
• Components of a colloidal solution:
  • Dispersed phase: The solute-like component or the dispersed particles.
  • Dispersion medium: The component in which the dispersed phase is suspended.

Physical and Chemical Changes

Physical Properties

• Properties that can be observed and specified (e.g., color, hardness, rigidity, fluidity, density, melting point, boiling point).
• Interconversion of states is a physical change because it occurs without a change in composition and no change in the chemical nature of the substance.
• Example: Ice, water, and water vapor are chemically the same, even though they have different physical properties.

Chemical Properties and Changes

• Chemical characteristics differentiate substances (e.g., cooking oil burns, water extinguishes fire).
• Burning is a chemical change in which a 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.
• During burning of a candle, both physical and chemical changes take place.

Types of Pure Substances

Elements

• Robert Boyle first used the term "element" in 1661.
• Antoine Laurent Lavoisier defined an element as a basic form of matter that cannot be broken down into simpler substances by chemical reactions.
• Elements are divided into metals, non-metals, and metalloids.
• Metals:
  • Have a lustre (shine).
  • Silvery-grey or golden-yellow color.
  • Conduct heat and electricity.
  • Ductile (can be drawn into wires).
  • Malleable (can be hammered into thin sheets).
  • Sonorous (make a ringing sound when hit).
  • Examples: gold, silver, copper, iron, sodium, potassium.
  • Mercury is the only metal that is liquid at room temperature.
• Non-metals:
  • Display a variety of colors.
  • Poor conductors of heat and electricity.
  • Not lustrous, sonorous, or malleable.
  • Examples: hydrogen, oxygen, iodine, carbon (coal, coke), bromine, chlorine.
• Metalloids:
  • Have intermediate properties between metals and non-metals.
  • Examples: boron, silicon, germanium.
  • The number of elements known at present are more than 100.
  • Ninety-two elements are naturally occurring and 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).

Compounds

• A compound is a substance composed of two or more elements chemically combined in a fixed proportion.
• When two or more elements are combined, they form a compound with different properties than the original elements.
• The composition of a compound is the same throughout.

Mixtures vs. Compounds

• Mixtures:
  1. Elements or compounds just mix together; no new compound is formed.
  2. Have variable composition.
  3. Show the properties of the constituent substances.
  4. Constituents can be separated fairly easily by physical methods.
• Compounds:
  1. Elements react to form compounds.
  2. Have fixed composition
  3. Constituents can only be separated by chemical or electrochemical reactions.