Matter and its properties
Chalk, ink and perfume look very different, yet each is "something". Break a chalk stick into tiny bits and every bit is still white. Pour a drop of perfume and its smell fills the room. That "something" which everything is made of is matter.
Matter (द्रव्य) is whatever an object is made of; it has mass (वस्तुमान) and occupies space (volume, आकारमान).
In the figure : In the figure matter made of only one constituent is a substance (gold, diamond, water). Matter made of two or more substances is a mixture (sea water, air, soil).
Rule 1: Properties of matter — Even when matter is divided into small particles, its properties (white colour of chalk, blue colour of ink, fragrance of perfume) remain the same. Mass is measured with a common balance. Volume is the space occupied.
- Easy : A chalk piece has mass and volume.
- Medium : A crushed chalk powder is still white, so properties stay the same.
- Exam-type : A 2 kg iron ball and a 2 kg cotton bundle have the same mass but different volumes.
Rule 2: Substance and mixture — A substance has only one constituent (gold, diamond, water, chalk). A mixture has two or more substances (sea water, air, soil).
- Easy : Water: substance.
- Medium : Sea water: mixture of water and salts.
- Exam-type : Tea with sugar is a mixture of tea, milk, water and sugar; the sugar alone is a substance.
Matter has mass and volume. Substance = one constituent; mixture = two or more.
Calling air a substance. Air is a mixture of gases.
Memory clue: "One = Substance, Many = Mixture".
Spot the difference
| Type | Constituents | Examples |
|---|---|---|
| Substance | One | Gold, water |
| Mixture | Two or more | Air, soil, sea water |
Practice questions
- 1What is matter?
- 2Name two properties of matter.
- 3What do we use to measure mass?
- 4What is a substance? Give an example.
- 5What is a mixture? Give an example.
- 6Classify as substance or mixture: diamond, soil, water, sea water.
- 7A 2 kg iron ball and a 2 kg cotton bundle: which has the larger volume, and why?
- 8Reena says crushed chalk is no longer chalk because its shape changed. Correct her.
Show answers (try them yourself first)
- 1Whatever an object is made of.
- 2Mass and volume.
- 3A common balance.
- 4Matter made of only one constituent, e.g. gold.
- 5Matter made of two or more substances, e.g. air.
- 6Substances: diamond, water. Mixtures: soil, sea water.
- 7The cotton; same mass but it is spread over more space.
- 8Its properties (white colour) remain the same, so it is still chalk.
Elements, atoms and symbols
A heap of bricks can build a house, a wall or a well, yet the brick itself is simple. In the same way, every substance is built from very small building blocks called atoms. Few kinds of atoms make a huge number of things.
An element (मूलद्रव्य) is a substance whose molecules are made of atoms that are exactly alike. It cannot be broken into different substances.
In the figure : In the figure 118 elements are known: 92 occur in nature and 26 are man-made (118 − 92 = 26).
Rule 1: Atoms and molecules — The smallest particles of substances are molecules. Atoms cannot be seen with the naked eye, and the mass and volume of atoms of different elements differ. Democritus named the small particles atom (Greek atomos = indivisible). In 1803 John Dalton proposed that atoms cannot be created, divided or destroyed. Two oxygen atoms join to form a molecule of oxygen.
- Easy : Oxygen molecule: 2 atoms of oxygen.
- Medium : 92 of the elements occur in nature; the rest, 118 − 92 = 26, are man-made.
- Exam-type : Dalton's idea came in 1803, which is 2026 − 1803 = 223 years before 2026.
In the figure : In the figure symbols are short forms of the names: C for carbon, Cl for chlorine, W for tungsten (German Wolfram); Ag and Au come from the Latin names Argentum and Aurum.
Rule 2: Symbols — Berzelius first used the present symbols: the short form of the name, written in English script (C for carbon, Cl for chlorine). When the initials are the same, a pair of letters is used.
- Easy : C is the symbol of carbon.
- Medium : Cl is the symbol of chlorine, since C is already used for carbon.
- Exam-type : Ag (silver) and Au (gold) come from the Latin Argentum and Aurum.
In the figure : In the figure a molecule of an element has atoms that are exactly alike (P: two oxygen atoms, R: two hydrogen atoms). Q is a molecule of water, whose atoms are of different types.
Rule 3: Element molecules — Examples of elements: hydrogen, oxygen, nitrogen, carbon, iron, mercury and copper. The molecule of an element has atoms that are exactly alike.
- Easy : O2 has 2 oxygen atoms.
- Medium : 3 molecules of O2 have 3 × 2 = 6 atoms.
- Exam-type : Water has atoms of different types (hydrogen and oxygen), so it is not an element.
Element: all atoms alike. 118 known = 92 natural + 26 man-made.
Thinking water is an element because it is a pure liquid. It has two kinds of atoms.
Memory clue: "E for Equal atoms": element = equal (alike) atoms.
Spot the difference
| Name | Symbol |
|---|---|
| Carbon | C |
| Chlorine | Cl |
| Gold (Aurum) | Au |
| Silver (Argentum) | Ag |
Practice questions
- 1What is an element?
- 2How many elements are known, and how many occur in nature?
- 3Who first used the present symbols for elements?
- 4What does the Greek word 'atomos' mean?
- 5What did Dalton propose in 1803?
- 6Write the symbols of carbon, chlorine, gold and silver.
- 7How many atoms are there in 3 molecules of oxygen (O2)?
- 8How many elements are man-made? Show the calculation.
Show answers (try them yourself first)
- 1A substance whose molecules are made of atoms that are exactly alike.
- 2118 known; 92 occur in nature.
- 3Berzelius
- 4Indivisible
- 5Atoms cannot be created, divided or destroyed.
- 6C, Cl, Au, Ag
- 73 × 2 = 6
- 8118 − 92 = 26
Metals, non-metals, metalloids and alloys
A steel spoon rings when struck, an aluminium foil can be pressed into a thin sheet and copper is drawn into long wires. A sulphur lump is dull and brittle. Elements differ in how they look and behave, so we group them.
Elements are classified as metals (धातू), non-metals (अधातू) and metalloids (धातुसदृश). An alloy (मिश्रधातू) is a mixture of metals.
In the figure : In the figure elements are metals (iron, copper, gold), non-metals (sulphur, phosphorus, chlorine) or metalloids (arsenic, silicon, selenium).
Rule 1: Metals, non-metals and metalloids — Metals show malleability, ductility, conductivity, density, lustre and sonority. Non-metals include phosphorus, sulphur and chlorine. Metalloids (arsenic, silicon, selenium) show some properties of both.
- Easy : Iron and copper are metals.
- Medium : Sulphur is a non-metal; silicon is a metalloid.
- Exam-type : An element that shines, can be drawn into wires and conducts electricity is most likely a metal.
In the figure : In the figure properties of metals: A malleability (beaten into sheets), B ductility (drawn into wires), C lustre (shine), D sonority (rings when struck).
Rule 2: Properties of metals — Malleability: beaten into sheets. Ductility: drawn into wires. Lustre: shine. Sonority: rings when struck.
- Easy : Gold is beaten into thin sheets: malleability.
- Medium : Copper is drawn into wires: ductility.
- Exam-type : A bell rings when struck: sonority.
In the figure : In the figure P: a pure metal has only one kind of atom. Q: an alloy is a mixture in which a metal is mixed with other elements to modify its properties.
Rule 3: Alloys and special symbols — A mixture of metals is an alloy: brass, steel, 22 carat gold. Pure iron rusts and pure gold is very soft, so their properties are modified by mixing other elements. The tungsten wire in a bulb has the symbol W (German Wolfram).
- Easy : Brass and steel are alloys.
- Medium : 22 carat gold is 22 parts gold out of 24, i.e. 22 ÷ 24 × 100 ≈ 91.7% gold.
- Exam-type : Jewellers add other metals to gold because pure gold is very soft.
Metals, non-metals, metalloids and alloys
In the app you can move this figure with your finger.
Open in the appMetals: malleable, ductile, lustrous, sonorous. Alloys = mixtures of metals.
Calling brass an element. Brass is an alloy, a mixture.
Memory clue: "MD-LS": Malleable, Ductile, Lustrous, Sonorous; alloys = mixed metals.
Spot the difference
| Class | Examples | Property |
|---|---|---|
| Metals | Iron, copper, gold | Malleable, ductile, lustrous |
| Non-metals | Sulphur, phosphorus, chlorine | Mostly dull and brittle |
| Metalloids | Arsenic, silicon, selenium | Some of both |
Practice questions
- 1Into which three classes are elements divided?
- 2Name two metals and two non-metals.
- 3What is malleability?
- 4What is ductility?
- 5What is an alloy? Give two examples.
- 6What is the symbol of tungsten and where does it come from?
- 7What percentage of 22 carat gold is pure gold (out of 24 parts)?
- 8Why is pure gold not used for making jewellery directly?
Show answers (try them yourself first)
- 1Metals, non-metals and metalloids.
- 2Metals: iron, copper. Non-metals: sulphur, chlorine.
- 3The property of being beaten into sheets.
- 4The property of being drawn into wires.
- 5A mixture of metals, e.g. brass and steel.
- 6W, from the German word Wolfram.
- 722 ÷ 24 × 100 ≈ 91.7%
- 8Pure gold is very soft, so other metals are mixed to make an alloy.
Compounds and mixtures
Salt dissolved in water and salt-and-pepper sprinkled together look similar, but they are different kinds of "together". In one, a new thing is formed; in the other, the two parts simply sit side by side.
A compound (संयुग) is formed by the chemical combination of two or more elements. A mixture (मिश्रण) is formed by mixing without chemical change.
In the figure : In the figure a molecule of water has 2 atoms of hydrogen and 1 atom of oxygen, so its molecular formula is H2O.
Rule 1: Compounds — The molecules of a compound are made of different types of atoms (water: 2 hydrogen + 1 oxygen). Heating sugar leaves black carbon. The properties of a compound differ from those of its elements: hydrogen burns, oxygen helps combustion, but water extinguishes fire. A molecular formula (रेणूसूत्र) is written using symbols and the number of atoms.
- Easy : H2O has 2 + 1 = 3 atoms.
- Medium : In 4 molecules of water there are 4 × 3 = 12 atoms: 8 hydrogen and 4 oxygen.
- Exam-type : Hydrogen burns and oxygen helps burning, but their compound, water, puts out fire.
In the figure : In the figure P: an element (one kind of atom). Q: a compound (different atoms chemically combined, all molecules alike). R: a mixture (no chemical change, atoms not combined, proportion can vary).
Rule 2: Mixtures — A mixture is formed by mixing elements or compounds. The proportion of the components is not fixed, no chemical change occurs and no new substance forms. Mixing unwanted substances in food is adulteration.
- Easy : Air is a mixture of gases.
- Medium : Salt and sand mixed in any ratio is a mixture, because the proportion is not fixed.
- Exam-type : Water is always 2 parts hydrogen to 1 part oxygen atoms (fixed), so it is a compound, not a mixture.
Compound: fixed ratio, new properties. Mixture: any ratio, same properties as parts.
Calling a mixture of salt and sand a compound. No chemical change happens in a mixture.
Memory clue: "Compound = Combined (chemically); Mixture = Mixed (just together)".
Spot the difference
| Point | Compound | Mixture |
|---|---|---|
| Chemical change? | Yes | No |
| Proportion | Fixed | Not fixed |
| Properties | New | Same as the parts |
| Example | Water (H2O) | Air, soil |
Practice questions
- 1What is a compound?
- 2Write the molecular formula of water.
- 3What is a molecular formula?
- 4What is a mixture?
- 5What is adulteration?
- 6How many atoms of each kind are in 4 molecules of water?
- 7Hydrogen burns and oxygen helps burning. Why can water put out fire?
- 8Classify as element, compound or mixture: oxygen, water, air, soil.
Show answers (try them yourself first)
- 1A substance formed by chemical combination of two or more elements.
- 2H2O
- 3A formula written with symbols and the number of atoms in a molecule.
- 4Elements or compounds mixed without chemical change and without a fixed proportion.
- 5Mixing unwanted substances in food.
- 6Hydrogen 4 × 2 = 8; oxygen 4 × 1 = 4.
- 7A compound has properties different from its elements.
- 8Element: oxygen. Compound: water. Mixtures: air, soil.
Methods of separating mixtures
The cook picks stones out of rice, the farmer winnows husk from grain, and the tea strainer keeps tea leaves out of your cup. Everyone separates mixtures in daily life, with a method that suits the mixture.
Separation (पृथक्करण) is dividing a mixture into its components using a method that suits the properties of the components.
In the figure : In the figure of distillation: salt water boils in the flask (A), the vapour cools and condenses in the sloping tube (B), pure water collects in the beaker (C) and the salt remains behind.
Rule 1: Simple methods and distillation — Simple methods: straining (filtering), sifting, picking, sorting, winnowing, combing with a magnet and sublimation. Distillation: salt water is boiled, the vapour condenses in a cooled tube and pure water collects while salt remains behind; it also purifies impure liquids.
- Easy : Winnowing separates husk from grain.
- Medium : A magnet picks iron filings from sand.
- Exam-type : Distillation gives pure water from salt water, but filtering would not, since salt is dissolved.
In the figure : In the figure of a separating funnel two liquids that do not mix form layers, the lighter kerosene above (A) and the heavier water below (B). Opening the tap (C) lets the lower liquid out.
Rule 2: Separating funnel — A separating funnel separates immiscible liquids (kerosene floats on water).
- Easy : Oil and water form two layers.
- Medium : Open the tap to let the heavier lower liquid out first.
- Exam-type : Cooking oil floats on water, so a separating funnel can separate them.
In the figure : In the figure of a centrifuge spinning (Q) makes the tiny solid particles collect at the bottom of the tube instead of staying spread out (P).
Rule 3: Centrifuge — A centrifuge spins test tubes so that tiny solid particles (milk, ink, blood) collect at the bottom.
- Easy : Blood cells collect at the bottom in a centrifuge.
- Medium : Cream can be separated from milk by spinning.
- Exam-type : Ordinary filtering fails for very tiny particles, so spinning is used.
In the figure : In the figure of chromatography the ink spot is put on filter paper. As the solvent rises, the colours rise to different heights (sample heights: red 2 cm, blue 4 cm, yellow 7 cm).
Rule 4: Chromatography — Chromatography separates substances dissolved in small proportions. It uses their solubility in the moving solvent and their sticking to the stationary filter paper. Components rise to different heights.
- Easy : Black ink separates into several colours on filter paper.
- Medium : If one colour reaches 3 cm and another 6 cm, the second rose twice as high (6 ÷ 3 = 2).
- Exam-type : A colour that sticks to the paper rises little; one that dissolves easily in the solvent rises higher.
Match the method to the mixture: solid-solid, liquid-liquid, dissolved solids, tiny solids.
Using filtering for salt dissolved in water. Dissolved salt passes through the filter paper.
Memory clue: "D-S-C-C": Distillation (salt water), Separating funnel (two liquids), Centrifuge (tiny solids), Chromatography (colours).
Spot the difference
| Method | Separates | Example |
|---|---|---|
| Distillation | Dissolved solid from liquid | Salt water |
| Separating funnel | Two immiscible liquids | Kerosene and water |
| Centrifuge | Tiny solid particles | Milk, blood |
| Chromatography | Small dissolved amounts | Ink colours |
Practice questions
- 1Name four simple methods of separating mixtures.
- 2Which method gives pure water from salt water?
- 3Which apparatus separates two liquids that do not mix?
- 4What does a centrifuge do?
- 5What does chromatography separate?
- 6Which liquid floats on water in a separating funnel?
- 7In a chromatography strip the colours rose to 2 cm, 4 cm and 7 cm. Which colour is the most soluble in the solvent?
- 8Suggest a method to separate (a) iron filings from sand, (b) cream from milk, and (c) black ink into colours.
Show answers (try them yourself first)
- 1Straining, sifting, picking and winnowing.
- 2Distillation
- 3Separating funnel
- 4Spins test tubes so tiny solid particles collect at the bottom.
- 5Substances dissolved in small proportions.
- 6Kerosene
- 7The one that rose 7 cm.
- 8(a) Magnet; (b) centrifuge; (c) chromatography.
Quick revision
- Element: one kind of atom; compound: different atoms chemically combined; mixture: no chemical change, proportion not fixed.
- Metals, non-metals and metalloids; alloys are mixtures of metals.
- Distillation, separating funnel, centrifugation and chromatography separate mixtures.