Mixtures and Pure Substances

• For a common person, 'pure' means unadulterated, but to a scientist, most everyday items are mixtures of different substances and therefore not pure
• A scientist considers a substance pure when all its constituent particles are identical in chemical nature
• A pure substance consists of a single type of particle
• Most matter around us exists as mixtures

What is a Mixture?

• Mixtures comprise more than one pure form of matter
• Dissolved sodium chloride can be separated from water by evaporation, but sodium chloride itself cannot be separated into its chemical constituents by physical means
• Sugar contains only one kind of pure matter with uniform composition
• Soft drinks and soil are not single pure substances
• A pure substance maintains the same characteristic properties regardless of its source
• A mixture contains more than one pure substance

Types of Mixtures

• Mixtures can be classified based on the nature of their components
• Homogeneous mixtures have a uniform composition throughout
• Examples include salt dissolved in water and sugar dissolved in water
• Homogeneous mixtures can have variable compositions
• Heterogeneous mixtures have physically distinct parts with non-uniform compositions
• Examples of heterogeneous mixtures include sodium chloride and iron filings, salt and sulfur, and oil and water

Activity (Mixtures)

• Divide a class into four groups (A, B, C, and D) and distribute samples
• Group A receives a few crystals of copper sulfate, Group B gets one spatula full of copper sulfate, Group C receives chalk powder or wheat flour, Group D gets a few drops of milk or ink
• Have each group add their sample to water, stir, and observe if the particles are visible
• Shine a beam of light through each mixture and observe from the front to see if the light's path is visible
• Let the mixtures sit undisturbed and observe if the particles settle over time, then filter each mixture and check for residue on the filter paper
• Groups A and B will have solutions, Group C will have a suspension, and Group D will have a colloidal solution

Questions

• A substance is matter with a uniform and definite composition
• Homogeneous mixtures have uniform composition and properties throughout (e.g., salt water), while heterogeneous mixtures have non-uniform composition with distinct phases (e.g., oil and water)

What is a Solution?

• A solution is a homogeneous mixture of two or more substances
• Examples found in daily life include lemonade and soda water
• Solutions can be liquid, solid (alloys), or gaseous (air)
• Solutions exhibit homogeneity at the particle level, with substances evenly distributed
• Alloys are mixtures of two or more metals/non-metals that cannot be separated by physical methods, but are still considered mixtures as they retain properties of their constituents and have variable composition
• Brass is an alloy of approximately 30% zinc and 70% copper

Solution Components

• A solution consists of a solvent and a solute
• The solvent dissolves the other component and is present in a larger amount
• The solute is dissolved in the solvent and is present in a smaller amount

Examples of Solutions

• Sugar in water (solid in liquid), where sugar is the solute and water is the solvent
• Tincture of iodine (iodine in alcohol), where iodine (solid) is the solute and alcohol (liquid) is the solvent
• Aerated drinks (gas in liquid), where carbon dioxide (gas) is the solute and water (liquid) is the solvent
• Air (gas in gas), a homogeneous mixture of gases with oxygen (21%) and nitrogen (78%) as the main constituents

Properties of a Solution

• Solutions are homogeneous mixtures
• Solution particles are smaller than 1 nm and cannot be seen with the naked eye
• Solutions do not scatter light
• Solute particles cannot be separated by filtration
• Solutions are stable, with solute particles not settling over time

Concentration of a Solution

• The relative proportion of solute and solvent in a solution can vary
• Solutions can be dilute, concentrated, or saturated depending on the amount of solute present
• Dilute and concentrated are comparative terms
• A is dilute compared to B if A and B are both solutions, but B contains more solute
• At a particular temperature, a saturated solution has dissolved as much solute as it is capable of dissolving
• Solubility is the amount of solute present in a saturated solution at a specific temperature
• An unsaturated solution contains less solute than the saturation level
• Different substances have different solubilities in a given solvent at the same temperature
• Concentration is mass/volume of solute present in a given amount of solution

Expressing Concentration Methods

• Mass by mass percentage= (Mass of solute / Mass of solution) * 100
• Mass by volume percentage= (Mass of solute / Volume of solution) * 100
• Volume by volume percentage = (Volume of solute / Volume of solution) * 100

Suspensions

• Non-homogeneous systems, where solids are dispersed in liquids, are called suspensions
• Suspensions are heterogeneous mixtures in which solute particles do not dissolve but remain suspended throughout the bulk of the medium
• Particles in a suspension are visible to the naked eye
• Suspensions are heterogeneous mixtures
• Suspension particles can be seen with the naked eye
• Suspension particles scatter a beam of light, making its path visible
• Suspensions are unstable, as solute particles settle when undisturbed
• Suspension components can be separated via filtration

Colloidal Solutions

• Colloids are mixtures where particles are uniformly spread throughout the solution
• Colloid particles are relatively smaller in size, compared to suspension
• Colloidal solutions appear homogeneous but are actually heterogeneous
• Colloid particles cannot be seen with the naked eye, but can scatter a beam of visible light
• The Tyndall effect is the scattering of light by colloid particles and is named after the scientist who discovered it

Tyndall Effect

• It can be observed when sunlight passes through the canopy of a dense forest where mist contains droplets of water that act as colloid particles dispersed in air
• Colloids are heterogeneous mixtures
• Colloid particles are too small to be seen individually with the naked eye
• Colloid particles are large enough to scatter light, making the beam visible
• Colloids do not settle when undisturbed (stable)
• Colloid components cannot be separated by filtration, but can be separated by centrifugation

Components a Colloidal Solution

• The components of a colloidal solution are the dispersed phase and the dispersion medium
• Dispersed phase particles are solute-like
• The dispersion medium is what the dispersed phase is suspended in
• Colloids are classified based on whether their dispersed medium is a solid, liquid, or gas

Physical and Chemical Changes

• Physical properties that can be observed include color, hardness, rigidity, fluidity, density, melting point, and boiling point
• Interconversion of states is a physical change as it does not alter chemical composition

Chemical Properties

• Chemical properties are related to a substance's chemical characteristics, like odor and inflammability
• Burning is a chemical change that alters chemical composition
• A chemical change is also called a chemical reaction
• Burning a candle involves both physical and chemical changes

Types of Pure Substances

• Pure 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 are normally divided into metals, non-metals, and metalloids

Properties of Metals

• Metals typically have: lustre (shine), silvery-grey/golden-yellow color, conduct heat/electricity, are ductile (can be drawn into wires), malleable (can be hammered), and sonorous (make ringing sound) - Gold, silver, copper, iron, sodium, and potassium are examples of metals.
• Mercury is the only metal that is liquid at room temperature

Non-Metals

• Non-metals display a variety of colors and are poor conductors of heat and electricity
• They are typically not lustrous, sonorous, or malleable
• Examples of non-metals include hydrogen, oxygen, iodine, carbon, and bromine

Metalloids

• Some elements have intermediate properties between those of metals and non-metals (metalloids)
• Examples include boron, silicon, and germanium
• There are over 100 known elements
• Ninety-two elements occur naturally
• The rest are human-made
• Most elements are solid
• Eleven elements are gases at room temperature
• Mercury and bromine are liquid elements at room temperature
• Gallium and cesium become liquid 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, a compound is formed

Activity: Iron and Sulfur

• Divide a class into two groups
• group A has iron filings and sulfur powder
• group B mixes, and strongly heats iron filings and sulfur powder until red hot
• Remove from flame and let the mixture cool
• Magnetism: Check magnetism in the material obtained by approaching it with a magnet
• Texture/Color: Compare the texture and color of the materials obtained
• Carbon Disulphide: Add carbon disulphide to one part of each material, stir well and filter
• Acid: Add dilute sulphuric or hydrochloric acid to another part of both materials
• Group A's material is a mixture of iron and sulphur
• Mixture properties are that of its constituent's material
• Group B's material is a compound comprised of iron + sulphur
• A compound's properties are totally different from its combined elements

Mixtures vs Compounds

• Mixtures:*
• Elements or compounds mix without forming new compounds
• Variable composition
• Show properties of constituent substances
• Constituents can be separated fairly easily by physical methods
• Compounds:*
• Elements react to form new compounds.
• The composition of each new substance is always fixed
• The substances have totally different properties
• Consituents can be separated only by chemical or electrochemical reactions
• The texture and color of a compound are the same throughout