Particulate Nature of Matter

7.1 What Is Matter Composed Of?

Matter consists of extremely small constituent particles. Grinding chalk changes the size of visible fragments but does not create a new substance. Sugar dissolves in water and becomes invisible to the eye, but its particles remain in solution. Constituent particles are much smaller than ordinary powder grains. Acharya Kanad proposed an early philosophical idea of parmanu; modern atomic science rests on experimental evidence.

7.2 What Decides Different States of Matter?

Interparticle attractions and thermal motion influence the state of matter. In solids particles remain closely arranged and vibrate about positions. In liquids particles stay close but move past one another. In gases particles are widely separated and move freely. These are simplified particle models, not pictures drawn to scale.

7.2.1 Solid State

Solids generally have fixed shape and volume. Strong interactions restrict movement to vibration. Heating often increases vibration until melting occurs at the melting point under given pressure. Ice melts near 0 °C, urea near 133 °C and iron near 1538 °C at atmospheric pressure, as in the textbook. Melting is a physical change.

7.2.2 Liquid State

Liquids have a definite volume but no fixed shape. In Activity 7.4, 200 mL of water is transferred between differently shaped containers: its shape changes but its volume remains approximately 200 mL if no water is lost. At boiling, vapour forms within the liquid and at the surface; evaporation occurs at the surface below boiling point too.

7.2.3 Gaseous State

Gases have neither fixed shape nor fixed volume and fill their available container. Their particles move in all directions and are comparatively far apart. The smoke demonstration makes spreading visible, but smoke contains suspended particles and is not itself a pure gas. Liquids and gases are called fluids because they flow.

7.3 Interparticle Spacing

A needle-free syringe containing trapped air can be compressed substantially because gas particles are far apart. Water is much less compressible. When sugar dissolves, the total solution volume need not equal the simple sum of water and dry sugar volumes; molecular interactions and packing matter. Sand is insoluble and settles instead of forming a molecular solution.

7.4 Movement of Particles

Potassium permanganate can spread through water without stirring, illustrating diffusion. It often spreads faster in warmer water due to faster molecular motion and convection. Fragrance spreads through air as particles move and air currents transport them. Diffusion is not the same as dissolving, and visible colour changes do not reveal individual molecules.

Real-Life Applications

Perfume reaches distant noses because volatile molecules move through air; soap molecules help disperse oily dirt through their water-loving and oil-loving regions; compressed air is used in tyres and cylinders; melting ice and boiling water illustrate changes of state. Changes in state do not turn water molecules into a different chemical substance.

Comparison of Three States

Melting Point Reference

Activities 7.1–7.9: Procedures, Observations and Conclusions

Activity 7.1: teacher-supervised grinding of chalk shows smaller pieces of the same material. 7.2: dissolving sugar in drinking water demonstrates invisible dissolved particles; do not taste laboratory mixtures. 7.3: compare the shapes of iron, rock salt, stone, wood, key and aluminium; do not hammer items without adult supervision. 7.4: transfer 200 mL water among differently shaped containers and compare shape and volume. 7.5: smoke spreads between jars, but avoid smoke exposure; a safe scent-free simulation may substitute. 7.6: compress trapped air in a needle-free syringe and compare with water. 7.7: mark water levels A, B and C before adding sugar, after adding it and after dissolution. 7.8: teacher handles potassium permanganate; observe spreading without touching or tasting. 7.9: observe fragrance spreading; avoid burning incense in poorly ventilated classrooms. Report actual results rather than invented measurements.

Diagrams and Revision

Figure 7.1 shows chalk grinding; 7.4 solid melting; 7.5 water in three vessels; 7.7 smoke diffusion; 7.9 syringe compression; 7.10 sugar-water level changes; 7.12 particle spacing; 7.13 coloured diffusion; 7.16 candle wax in three states. For particle sketches, draw solids as close regularly arranged circles, liquids as close irregular circles and gases as widely spaced circles. These are schematic models. Common error: saying particles of a solid never move; they vibrate.

NCERT Keep the Curiosity Alive — full questions and solutions

Original practice questions with solutions

MCQs, HOTS and quick revision

MCQ: Which state has no fixed shape or volume? Gas. MCQ: Which is easier to compress: air or water? Air. HOTS: Why is ice less dense than liquid water despite being solid? Hydrogen bonding produces an open crystalline structure in ice, an exception to the common closer-packing rule. Exam tip: Never write that solid particles are motionless; they vibrate.

Discover, Design and Debate

Use clay beads to model solid, liquid and gas arrangements; compare the inflation of a balloon on a bottle in safely warmed water with adult supervision; role-play moving particles at different temperatures; discuss benefits and harms of gas spreading. Do not use boiling water, iodine vapour, open flames or potassium permanganate without a trained teacher.

Source, diagrams and limitations

Official NCERT Curiosity Chapter 7 PDF. All ten numbered exercises and the nine activities are covered. Figure 7.16 is explained conceptually; exact printed diagram letters require visual matching on the original figure. Particle diagrams are described in words, not reproduced as images.