Notes on Is Matter Around Us Pure?

Purity in Science vs. Common Usage

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

Mixtures

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 sodium chloride from water).
A pure substance contains only one kind of matter, with consistent composition throughout.
Mixtures contain more than one pure substance.

Types of Mixtures

Mixtures can be homogeneous or heterogeneous, depending on the uniformity of composition.

Homogeneous Mixtures (Solutions)

Have a uniform composition throughout.
Examples: salt dissolved in water, sugar dissolved in water.
Homogeneous mixtures can have variable compositions (e.g., copper sulphate solutions with different intensities of color).

Heterogeneous Mixtures

Contain physically distinct parts with non-uniform compositions.
Examples: mixtures of sodium chloride and iron filings, salt and sulphur, oil and water.

Activity 2.2 Observations

Group A & B: Solutions.
Group C: Suspension.
Group D: Colloidal solution.

Solutions

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

Mixtures of two or more metals or a metal and a non-metal.
Cannot be separated into components by physical methods.
Considered mixtures because they show properties of constituents and have variable composition.
Example: brass (approximately 30% zinc and 70% copper).

Components of a Solution

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 of Solutions

Sugar in water: solid in liquid (sugar is solute, water is solvent).
Tincture of iodine: iodine in alcohol (iodine is solute, alcohol is solvent).
Aerated drinks: gas in liquid (carbon dioxide is solute, water is solvent).
Air: gas in gas (oxygen and nitrogen are main constituents).

Properties of a Solution

Homogeneous mixture.
Particle size is less than $1 nm$ ( $10^{-9}$ meter).
Particles are not visible to the naked eye.
Do not scatter light passing through the solution; hence, the path of light is not visible.
Solute particles cannot be separated by filtration.
Solute particles do not settle down; the solution is stable.

Concentration of a Solution

The relative proportion of solute and solvent can vary.
Classified as dilute, concentrated, or saturated, depending on the amount of solute present.
Dilute and concentrated are comparative terms.
Saturated Solution: A solution that has dissolved as much solute as it is capable of dissolving at a particular temperature. No more solute can be dissolved.
Solubility: The amount of solute present in a saturated solution at a specific temperature.
Unsaturated Solution: A solution containing less solute than the saturation level.
Different substances have different solubilities in a given solvent at the same temperature.
The concentration of a solution is the amount of solute present in a given amount 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 2.1

A solution contains 40 g of common salt in 320 g of water.
Mass of solute (salt) = 40 g
Mass of solvent (water) = 320 g
Mass of solution = 40 g + 320 g = 360 g
Mass percentage of solution = $\frac{40}{360} \times 100 = 11.1\%$

Suspensions

Non-homogeneous systems in which solids are dispersed in liquids.
Heterogeneous mixture where solute particles do not dissolve but remain suspended throughout the medium.
Particles are visible to the naked eye.

Properties of a Suspension

Heterogeneous mixture.
Particles can be seen with the naked eye.
Scatter a beam of light passing through it, making the path visible.
Solute particles settle down when undisturbed (unstable).
Can be separated by filtration.
Suspension breaks down and does not scatter light when particles settle.

Colloidal Solutions

The particles are uniformly spread throughout the solution.
Appear homogeneous but are actually heterogeneous mixtures (e.g., milk).
Particles are small and cannot be seen with naked eyes but can scatter light.

Tyndall Effect

Scattering of a beam of light by colloidal particles.
Observed when a fine beam of light enters a room through a small hole due to dust and smoke particles.
Colloids are big enough to scatter light and make its path visible.
Do not settle down when left undisturbed (quite stable).
Cannot be separated by normal filtration but can be separated using centrifugation.

Components of Colloidal Solutions

Dispersed phase: Solute-like component or dispersed particles.
Dispersion medium: The component in which the dispersed phase is suspended.

Classification of Colloids

Classified based on the state (solid, liquid, or gas) of the dispersed phase and dispersing medium.

Common Examples of Colloids

Aerosol: Liquid in gas (fog, clouds, mist), Solid in gas (smoke, automobile exhaust).
Foam: Gas in liquid (shaving cream), Gas in solid (foam, rubber, sponge, pumice).
Emulsion: Liquid in liquid (milk, face cream).
Sol: Solid in liquid (milk of magnesia, mud).
Gel: Liquid in solid (jelly, cheese, butter).
Solid Sol: Solid in solid (coloured gemstone, milky glass).
Tyndall effect can be observed when sunlight passes through the canopy of a dense forest due to water droplets in the mist.

Physical and Chemical Changes

Physical Properties

Properties that can be observed and specified: colour, hardness, rigidity, fluidity, density, melting point, boiling point, etc.
Interconversion of states is a physical change because it occurs without a change in composition or chemical nature.
Example: Ice, water, and water vapour are chemically the same despite different physical properties.

Chemical Properties

Determine how a substance reacts with others (e.g., burning of oil vs. water).
Chemical change involves a change in chemical composition, resulting in new substances.
A chemical change is also called a chemical reaction.

Types of Pure Substances

Substances can be classified as elements or compounds based on their chemical composition.

Elements

Robert Boyle was the first scientist to use 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 can be divided into metals, non-metals, and metalloids.

Metals

Have lustre (shine).
Silvery-grey or golden-yellow colour.
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 colours.
Poor conductors of heat and electricity.
Not lustrous, sonorous, or malleable.
Examples: hydrogen, oxygen, iodine, carbon, bromine, chlorine.

Metalloids

Have intermediate properties between metals and non-metals.
Examples: boron, silicon, germanium.

Compounds

A substance composed of two or more elements chemically combined in a fixed proportion.
Elements react to form new compounds.

Activity 2.4

Group I: Mixing iron filings and sulphur powder (physical change).
Group II: Heating iron filings and sulphur powder (chemical change).
Group I obtains a mixture with magnetic properties; Group II obtains a compound with different properties.
The gas obtained by Group I (adding dilute acid) is hydrogen, and the gas obtained by Group II is hydrogen sulphide.

Key Points

The material obtained by Group I is a mixture of iron and sulphur; the properties of the mixture are the same as that of its constituents.
The material obtained by Group II is a compound with totally different properties compared to the combining elements.
The composition of a compound is the same throughout.
More than 100 elements are known; 92 are naturally occurring, and the rest are man-made.
Majority of the elements are solid.
Eleven elements are gaseous at room temperature.
Two elements are liquid at room temperature—mercury and bromine.
Gallium and cesium become liquid at slightly above room temperature (303 K).

Mixtures vs. Compounds Comparison

Mixtures: Elements or compounds mix together without forming a new compound; have a variable composition; show properties of constituent substances; constituents can be separated fairly easily by physical methods.
Compounds: Elements react to form new compounds; have a fixed composition; have totally different properties than constituents; constituents can be separated only by chemical or electrochemical reactions.