States of matter and intermolecular force
A stone stays in your hand, water takes the shape of your glass and the smell of food spreads through the whole kitchen. The same matter behaves differently because its particles hold each other with different strength.
The intermolecular force of attraction (आंतरआण्विक आकर्षण बल) is the force acting between the particles of matter. Its strength decides whether matter is a solid, liquid or gas.
In the figure : In the figure solid particles are packed tightly and only vibrate. Liquid particles are close but move about. Gas particles are far apart and move freely.
Rule 1: Three states — Solid: strong force, particles close and vibrating at fixed places, so definite shape and volume. Liquid: moderate force, particles move about, so definite volume but no definite shape. Gas: very weak force, particles move freely and fill all space, so no definite shape or volume and it is highly compressible.
- Easy : A brick keeps its shape: solid.
- Medium : Juice takes the shape of the glass but its volume stays the same: liquid.
- Exam-type : A balloon is blown up with gas, which spreads to fill the whole balloon: gas.
In the figure : In the figure the intermolecular force is strongest in a solid, moderate in a liquid and weakest in a gas, shown by relative sample values.
Rule 2: Force and distance — The more energy particles get, the more they move. The distance between particles is minimum in a solid, moderate in a liquid and very large in a gas. So solids are not compressible, while gases can be compressed.
- Easy : Gas stored in a cooking cylinder is compressed.
- Medium : A cycle-tube pump squeezes air into a small space because the particles are far apart.
- Exam-type : You cannot squeeze a steel ball smaller, because its particles are already very close.
States of matter and intermolecular force
In the app you can move this figure with your finger.
Open in the appForce: solid strongest, gas weakest. Distance: solid least, gas most.
Saying that gas particles have no force at all. The force is very weak, not zero.
Memory clue: "Solid = Shut tight; Liquid = Loose; Gas = Gone free".
Spot the difference
| Point | Solid | Liquid | Gas |
|---|---|---|---|
| Force | Strong | Moderate | Very weak |
| Shape | Definite | Takes container's | None |
| Volume | Definite | Definite | Not definite |
Practice questions
- 1What is the intermolecular force of attraction?
- 2In which state is the force strongest?
- 3Which state has no definite shape and no definite volume?
- 4Why does a liquid take the shape of its container?
- 5Why can gases be compressed but solids cannot?
- 6Compare the distance between particles in the three states.
- 7Why does the smell of food spread in a kitchen but a stone does not spread?
- 8A student says liquids have no intermolecular force. Correct this.
Show answers (try them yourself first)
- 1The force of attraction between the particles of matter.
- 2Solid
- 3Gas
- 4Its particles are held together but not fixed, so they move about.
- 5Gas particles are far apart, solid particles are already very close.
- 6Solid: minimum; liquid: moderate; gas: very large.
- 7Gas particles move freely and fill all space; stone particles only vibrate at fixed places.
- 8Liquids have moderate force, which holds particles together but lets them move, giving a definite volume.
Element, compound and mixture
Tap water, steel spoon and juice look similar to a child as 'things'. But a chemist asks what they are made of. Pure oxygen has only oxygen atoms; juice has many things mixed.
Matter can be classified by chemical composition into elements (मूलद्रव्ये), compounds (संयुगे) and mixtures (मिश्रणे).
In the figure : In the figure an element has one kind of atom, a compound has different atoms bonded together, and in a mixture the different particles are not bonded.
Rule 1: Element, compound, mixture — Element: all atoms identical (O2, N2). Compound: two or more kinds of atoms joined by chemical bonds in a fixed proportion (H2O); its properties differ from its elements. Mixture: two or more substances not joined by bonds, proportion can change, each keeps its properties (air, brass, milk).
- Easy : Iron (Fe) is an element; carbon dioxide (CO2) is a compound; lemon sherbet is a mixture.
- Medium : Water is a compound and its properties differ from hydrogen and oxygen.
- Exam-type : Brass is a mixture of copper and zinc, with the proportion changing in different brass.
In the figure : In the figure pure water always has oxygen and hydrogen in the ratio 8 : 1 by mass, so a 9 g sample has 8 g of oxygen and 1 g of hydrogen.
Rule 2: Fixed proportion — In a compound the proportion is fixed. In pure water oxygen : hydrogen = 8 : 1 by mass, whatever the source.
- Easy : An 18 g sample of water has 16 g of oxygen and 2 g of hydrogen (16 : 2 = 8 : 1).
- Medium : In 27 g of water: total parts = 8 + 1 = 9. Hydrogen = 27 ÷ 9 = 3 g, oxygen = 3 × 8 = 24 g.
- Exam-type : If a sample has 40 g of oxygen, hydrogen is 40 ÷ 8 = 5 g, so the water is 45 g.
In the figure : In the figure fat is 3 to 5 percent in cow milk and 6 to 9 percent in buffalo milk.
Rule 3: Milk is a mixture — Milk contains water, lactose, fats and proteins. The proportion changes with the animal: fat is 3 to 5 percent in cow milk and 6 to 9 percent in buffalo milk. This changing proportion shows that milk is a mixture.
- Easy : 100 g of buffalo milk at 7 percent fat has 7 g of fat.
- Medium : 500 g of cow milk at 4 percent fat has 500 × 4 ÷ 100 = 20 g of fat.
- Exam-type : Buffalo milk (8 percent) and cow milk (4 percent), 250 g each: fat = 20 g and 10 g, so buffalo milk has 10 g more.
Element: same atoms. Compound: bonded, fixed ratio. Mixture: not bonded, ratio changes.
Thinking air is a compound. Air is a mixture because its gases are not bonded and the proportion can change.
Memory clue: "Element = Ek (one) kind; Compound = Chain (bonded); Mixture = Mela (a crowd, just together)".
Spot the difference
| Point | Element | Compound | Mixture |
|---|---|---|---|
| Particles | Same atoms | Different atoms bonded | Different, not bonded |
| Proportion | Not applicable | Fixed | Can change |
| Example | O2, N2 | H2O | Air, brass, milk |
Practice questions
- 1What is an element?
- 2Give two examples of mixtures.
- 3What is the ratio of oxygen to hydrogen in water by mass?
- 4How much oxygen is in 18 g of water?
- 5What is the main difference between a compound and a mixture?
- 6Find hydrogen and oxygen in 36 g of water.
- 7How much fat is in 400 g of cow milk with 5 percent fat?
- 8A sample contains 56 g of oxygen. How much water is it, if it is pure water?
Show answers (try them yourself first)
- 1A substance made of identical atoms.
- 2Air and brass (or milk).
- 38 : 1
- 416 g (18 ÷ 9 × 8)
- 5A compound has bonded atoms in a fixed ratio; a mixture has no bonds and the ratio can change.
- 6Hydrogen = 36 ÷ 9 = 4 g; oxygen = 4 × 8 = 32 g.
- 7400 × 5 ÷ 100 = 20 g
- 8Hydrogen = 56 ÷ 8 = 7 g; water = 56 + 7 = 63 g.
Types of elements and compounds
A steel knife is shiny and can be hammered, but a piece of coal is dull and breaks. Dry leaves burn leaving black ash, while salt on a hot pan only jumps and stays white. These differences help us sort elements and compounds.
Elements are metals, non-metals or metalloids. Compounds are organic, inorganic or complex.
Rule 1: Types of elements — Elements differ in lustre and malleability. Iron, copper and aluminium are shiny and flatten on hammering: metals. Coal (carbon) is dull and breaks: a non-metal. A third type is the metalloid.
- Easy : A copper wire is shiny and bends without breaking: metal.
- Medium : A sulphur lump is yellow, dull and breaks easily: non-metal.
- Exam-type : An aluminium foil is rolled out very thin by hammering-like pressing: malleable metal.
In the figure : In the figure an organic compound on strong heating leaves a black carbon residue or none, while an inorganic compound leaves a residue.
Rule 2: Organic and inorganic compounds — Organic compounds are carbon compounds such as carbohydrates, proteins, hydrocarbons (petrol, cooking gas), sugar, glucose, camphor and urea. On strong heating they leave a black carbon residue or none. Inorganic compounds such as common salt, soda, rust, blue vitriol and limestone decompose and leave a residue.
- Easy : Sugar turns black on heating: organic.
- Medium : Camphor burns away leaving no residue: organic.
- Exam-type : Limestone leaves a residue on heating: inorganic.
In the figure : In the figure complex compounds have a metal atom at the centre: magnesium in chlorophyll, iron in hemoglobin and cobalt in vitamin B-12.
Rule 3: Complex compounds — In a complex compound a metal atom sits at the centre of a large structure. Chlorophyll has magnesium, hemoglobin has iron and vitamin B-12 has cobalt.
- Easy : Hemoglobin has iron.
- Medium : Chlorophyll has magnesium and makes leaves green.
- Exam-type : Vitamin B-12 has cobalt at its centre.
Metals are shiny and malleable. Organic compounds contain carbon. Complex compounds have a central metal.
Thinking carbon dioxide and limestone are organic just because they contain carbon. Organic compounds here means carbohydrates, proteins, hydrocarbons and similar; limestone is inorganic.
Memory clue: "Organic = Out (burns or blackens); Inorganic = In (residue stays)".
Spot the difference
| Type | Key feature | Example |
|---|---|---|
| Metal | Shiny, malleable | Iron, copper |
| Non-metal | Dull, breaks | Coal, sulphur |
| Organic compound | Carbon residue or none | Sugar, urea |
| Inorganic compound | Leaves residue | Salt, limestone |
Practice questions
- 1Name two metals.
- 2Give an example of a non-metal.
- 3What is left on strongly heating an organic compound?
- 4Name two organic compounds.
- 5Name two inorganic compounds.
- 6What is a complex compound?
- 7Which metal is at the centre of chlorophyll, hemoglobin and vitamin B-12?
- 8A white powder on strong heating turns black and gives a burning smell. Is it likely organic or inorganic? Why?
Show answers (try them yourself first)
- 1Iron and copper
- 2Coal (carbon)
- 3A black carbon residue or nothing.
- 4Sugar and urea
- 5Common salt and limestone
- 6A compound with a metal atom at the centre of a complex structure.
- 7Magnesium, iron and cobalt.
- 8Organic, because organic compounds leave a black carbon residue on strong heating.
Types of mixtures
Sugar dissolved in tea is invisible, but sand in water settles down. Milk looks the same everywhere but light shines through fog differently. The size and spread of the particles decide how a mixture looks.
A phase (प्रावस्था) is a part of matter with uniform composition. A mixture with one phase is homogeneous (समांगी); with two or more phases it is heterogeneous (विषमांगी).
In the figure : In the figure a mixture with one phase is homogeneous (sugar in water) and a mixture with two or more phases is heterogeneous (oil and water, sand and water).
Rule 1: Homogeneous and heterogeneous — Sugar in water has one phase: homogeneous. Oil and water form separate layers: heterogeneous. All gases together make one phase.
- Easy : Salt solution looks the same everywhere: homogeneous.
- Medium : Mud in water has two phases: heterogeneous.
- Exam-type : Air is a mixture of gases but has one phase: homogeneous.
In the figure : In the figure a solute can be dissolved in a solvent in many state combinations: sugar syrup (solid in liquid), vinegar (liquid in liquid), air (gas in gas), brass (solid in solid) and HCl in water (gas in liquid).
Rule 2: Solutions — A solution is a homogeneous mixture. The component in the largest amount is the solvent (विद्रावक); the rest are solutes (द्राव्ये). Types: solid in liquid (sugar syrup, sea water), liquid in liquid (vinegar), gas in gas (air), solid in solid (brass, steel), gas in liquid (hydrochloric acid).
- Easy : In sugar syrup, water is the solvent and sugar is the solute.
- Medium : In soda water, carbon dioxide gas is dissolved in water: gas in liquid.
- Exam-type : In 100 g of salt solution with 5 g of salt, solvent (water) = 100 − 5 = 95 g.
In the figure : In the figure the particles of a suspension are larger than 10-4 m, while the particles of a colloid are about 10-5 m.
Rule 3: Suspension and colloid — A suspension is heterogeneous: particles larger than 10-4 m, light does not pass through, and the solid stays on the filter paper. A colloid is heterogeneous: particles about 10-5 m, translucent, and cannot be separated by ordinary filtration (milk; smoke, fog, clouds).
- Easy : Muddy water is a suspension; the mud settles.
- Medium : Milk is a colloid; it cannot be separated by ordinary filter paper.
- Exam-type : 10-4 m is 10 times larger than 10-5 m, which is why suspension particles settle.
Solution: homogeneous. Suspension and colloid: heterogeneous. Solvent is in the largest amount.
Saying a colloid is a solution because it looks uniform. A colloid is heterogeneous.
Memory clue: "Solution = Sugar (hidden); Suspension = Sinks; Colloid = Cloud (floats, scatters light)".
Spot the difference
| Point | Solution | Suspension | Colloid |
|---|---|---|---|
| Type | Homogeneous | Heterogeneous | Heterogeneous |
| Particle size | Very small | Over 10-4 m | About 10-5 m |
| Example | Sugar syrup, air | Muddy water | Milk, fog |
Practice questions
- 1What is a phase?
- 2What is a solution?
- 3Which component of a solution is the solvent?
- 4Name a gas-in-liquid solution.
- 5Name a solid-in-solid solution.
- 6Compare a suspension and a colloid in particle size.
- 7Why can a suspension be filtered but a colloid cannot?
- 8A 250 g salt solution has 15 g of salt. Find the amount of the solvent and the percent of salt.
Show answers (try them yourself first)
- 1A part of matter with uniform composition.
- 2A homogeneous mixture.
- 3The one in the largest amount.
- 4Hydrochloric acid (or soda water).
- 5Brass or steel
- 6Suspension particles are larger than 10-4 m; colloid particles are about 10-5 m.
- 7Suspension particles are larger and stay on the filter paper; colloid particles are smaller and pass through.
- 8Solvent = 250 − 15 = 235 g; percent of salt = 15 ÷ 250 × 100 = 6 percent.
Mixture versus compound, molecular formula and valency
Mix tea powder and sugar in a plate: you can pick each out again. But when you cook rice with water, you cannot get the raw rice back. A mixture can be separated by simple methods; a compound is a new thing.
A molecular formula (रेणुसूत्र) shows the symbols of the elements in a molecule and how many atoms of each. Valency (संयुजा) is the number of bonds an atom forms.
In the figure : In the figure a magnet attracts the iron in a mixture of iron and sulphur. After strong heating they form a compound and the magnet has no effect.
Rule 1: Mixture versus compound — Iron filings and sulphur mixed together: a mixture; the magnet still pulls the iron and yellow sulphur is visible. On heating they form a new compound: the colour changes and the magnet has no effect.
- Easy : Sand and sugar mixed in a plate: a mixture; each can be separated.
- Medium : Iron and sulphur mixed: a magnet separates the iron.
- Exam-type : After heating, iron sulphide is formed and the magnet no longer attracts it: a compound.
In the figure : In the figure valency is the number of bonds an atom forms: H and Cl have 1, O has 2, N has 3 and C has 4 (HCl, H2O, NH3, CH4).
Rule 2: Valency — Hydrogen is univalent (valency 1). From HCl, H2O, NH3 and CH4 the valencies of Cl, O, N and C are 1, 2, 3 and 4.
- Easy : In H2S, sulphur is bonded to 2 hydrogens, so its valency is 2.
- Medium : In PH3, phosphorus is bonded to 3 hydrogens, so valency 3.
- Exam-type : In SiH4, silicon is bonded to 4 hydrogens, so valency 4.
In the figure : In the figure, to write a formula, put the valencies side by side, cross-multiply and divide by the common number: Si (4) and O (2) give SiO2.
Rule 3: Writing formulae by cross multiplication — Write the symbols with their valencies, cross the valencies to be the subscripts, then divide by the common number. C (4) and O (2) give C2O4, which simplifies to CO2.
- Easy : Hydrogen (1) and chlorine (1) give H1Cl1, written HCl.
- Medium : Calcium (2) and chlorine (1) give CaCl2.
- Exam-type : Aluminium (3) and oxygen (2) give Al2O3. For magnesium (2) and oxygen (2): Mg2O2, divided by 2 gives MgO.
Mixture: separable by simple means. Compound: new substance. Cross multiply valencies and simplify.
Forgetting to simplify, writing Mg2O2 instead of MgO.
Memory clue: "Cross it, then Cut it": cross the valencies, then cut by the common number.
Spot the difference
| Point | Mixture | Compound |
|---|---|---|
| Formation | Just mixed | Chemical change |
| Properties | Each keeps its own | New properties |
| Separation | Simple methods, like magnet | Only by chemical methods |
Practice questions
- 1What is a molecular formula?
- 2What is valency?
- 3What is the valency of hydrogen?
- 4What is the valency of oxygen?
- 5Does a magnet attract iron in a mixture of iron and sulphur?
- 6Why does the magnet not attract the iron after strong heating?
- 7Write the formula of calcium chloride by cross multiplication.
- 8Write the formula for sodium sulphide, if Na has valency 1 and S has valency 2. Show the steps.
Show answers (try them yourself first)
- 1It shows the symbols of the elements and the number of atoms of each in a molecule.
- 2The number of chemical bonds an atom forms.
- 31
- 42
- 5Yes
- 6A new compound (iron sulphide) has formed.
- 7Ca (2), Cl (1): CaCl2
- 8Cross: Na2S1, written Na2S.
Quick revision
- Element: one kind of atom. Compound: bonded atoms, fixed proportion. Mixture: not bonded, proportion can change.
- Solution, suspension and colloid are the types of mixtures.
- Valency = number of bonds an atom forms; hydrogen has 1.