7.3 Solid State of Matter
🎯 Learning outcomes
By the end of this chapter, students should be able to:
- Differentiate between amorphous and crystalline solids.
- Describe the properties of crystalline solids (anisotropy, allotropy, isomorphism, polymorphism, transition temperature, habit of crystal, crystal growth).
- Define unit cell, crystal lattice, efflorescence, deliquescence, hygroscopy, and water of crystallisation with examples.
Ans: Solids have a fixed shape because their constituent particles are closely packed and held strongly in fixed positions. They can only vibrate about their mean positions.
Solids can be classified based on the dominant type of bonds they involve, viz:
- a. Ionic solids — e.g., NaCl
- b. Molecular solids — e.g., I₂
- c. Covalent solids — e.g., diamond
- d. Metallic solids — e.g., Cu
(as studied in detail in Chemical bonding and shape of molecules)
1. What are ionic crystals? Give examples.
Ans: A crystal in which ions are arranged in a specific pattern to give a certain structure is called an ionic crystal. E.g. NaCl, KNO₃, KCl, etc.
2. How does a covalent crystal differ from an ionic crystal?
| Covalent crystal | Ionic crystal |
|---|---|
| The crystal in which covalent bonds hold atoms together is called a covalent crystal, e.g., diamond, graphite, silica, etc. | A crystal in which ions are arranged in a specific pattern to give a certain structure is called an ionic crystal: E.g., NaCl, KNO₃, KCl, etc. |
3. What are molecular crystals? Give examples.
Ans: Molecular crystals contain molecules as their constituent particles, held together by relatively weak intermolecular forces. E.g., phosphorus, iodine, etc.
4. Metallic crystals are good conductors of heat and electricity. Justify the statement.
Ans: The constituents of metallic crystals are metallic ions in the pool of valence electrons where kernels are embedded. Due to the presence of these free electrons, metallic crystals are good conductors of heat and electricity.
5. NaCl is a bad conductor in the solid state but a good conductor in the molten state. Explain.
Ans: In the solid state, Na⁺ and Cl⁻ ions are held strongly by ionic interaction, so they are immobile, but in the molten state, those ions are mobile in nature. So NaCl is a bad conductor in the solid state but a good conductor in the molten state.
6. Why are the molecular crystals soft and volatile?
Ans: Due to weak Van der Waals interactions among constituent molecules, molecular crystals are volatile and soft in nature.
| Amorphous solid | Crystalline solid 💎 |
|---|---|
| Amorphous solids lack long-range regular arrangement of their constituent particles. They are sometimes described as supercooled liquids because of their structural similarity to liquids. | Crystalline solids possess a certain geometrical shape, and molecules are arranged in a definite pattern within it. |
| They generally melt at a range of temperatures. | They have sharp melting points. |
| They do not have sharp edges and plane surfaces. | Crystalline solids can develop well-defined plane surfaces and definite geometrical shapes. |
| They are isotropic. | They are anisotropic. |
1. Anisotropy
An isotropic solid is a solid material in which physical properties do not depend on the orientation. E.g. Glass
When the properties of a solid vary with different crystallographic orientations, the material is said to be an anisotropic solid. E.g. NaCl crystal
2. Allotropy
The existence of an element in two or more different forms in the same physical state is called allotropy.
Example: Carbon → diamond, graphite.
3. Isomorphism
The phenomenon in which different substances crystallise in the same or similar crystal structure is called isomorphism.
Example: Na₂SO₄ and K₂SO₄.
4. Polymorphism
The ability of a substance to exist in more than one crystalline form is called polymorphism.
Example: CaCO₃ → calcite and aragonite.
5. Transition Temperature 🌡️
The temperature at which one crystalline form changes into another crystalline form is called the transition temperature.
An example could be: Sulphur: rhombic ⇌ monoclinic sulphur at about 95.5°C.
6. Habit of Crystal
The characteristic external shape in which a crystal grows is called its crystal habit.
Examples: needle-shaped, cubic, prismatic, etc.
Efflorescence
Efflorescence is the spontaneous loss of some or all water of crystallisation from a hydrated salt when exposed to dry air. Such a substance is said to be efflorescent.
E.g., decahydrated sodium carbonate (Na₂CO₃.10 H₂O) loses 9 water molecules of water of crystallisation on exposure to form a monohydrated salt.
Other examples are FeSO₄.7 H₂O, CuSO₄.5H₂O, etc.
Hygroscopy
Certain solids absorb water molecules from the air and get hydrated. This is known as hygroscopy. Such solids are called hygroscopic solids. This is due to the lower vapour pressure of such a substance than the vapour pressure of the atmosphere.
For example, anhydrous copper sulphate (II) exposed to air becomes pentahydrated salt.
Deliquescence
Some solids absorb moisture from the atmosphere to an extent that they become wet and even change into their solution in some instances. Such solids are said to be deliquescent solids, and the phenomenon is called deliquescence.
This is also due to the lower vapour pressure of the substance than the vapour pressure of the atmosphere, e.g., anhydrous CaCl₂, NaOH, KOH, etc.
CuSO₄ absorbs water and turns blue but does not dissolve; CaCl₂ absorbs so much water that it forms a solution.
| Phenomenon | What happens? | Example |
|---|---|---|
| 🌬️ Hygroscopy | Absorbs moisture from air but does not necessarily form a solution | CuSO₄ |
| 💧 Deliquescence | Absorbs so much moisture that it dissolves and forms a solution | CaCl₂ |
| 💨 Efflorescence | Loses water of crystallisation to air | Na₂CO₃·10H₂O |
Crystallisation is the slow precipitation (Precipitation is the formation of a solid in a solution during a chemical reaction or by diffusion in a solid) of crystals from a solution of a substance.
Crystallisation is the process of obtaining pure crystals of a substance from its solution.
A hot saturated solution is allowed to cool slowly. As the solubility decreases, crystals separate from the solution.
The crystallisation point is the temperature at which a saturated solution, when heated, is cooled to obtain crystals.
Crystals have their constituent particles arranged regularly in short- and long-range in fixed spots in the crystal lattice. But the constituent particles of liquids or solutions are irregularly arranged and moving continuously throughout the body of the liquid. The crystallisation process involves the arrangement of the particles in a regular pattern from the solution to crystals in a stepwise manner. This process is called crystal growth.
During crystallisation, particles arrange themselves in a regular repeating pattern. As more particles join the crystal lattice, the crystal becomes larger. This process is called crystal growth. 🌱
Some crystalline salts contain a fixed number of water molecules in their crystal structure. Such salts are called hydrated salts.
Water of crystallisation is the fixed number of water molecules chemically associated with one formula unit of a crystalline salt.
Crystals having water of crystallisation are called hydrated salts, and those without water of crystallisation are called anhydrous salts.
Water of crystallisation serves as an integral part of the crystal and determines the geometry of the crystals and other properties in some cases.
| Molecular formula | Name | Common name |
|---|---|---|
| CuSO₄.5H₂O | Copper sulphate pentahydrate | Blue vitriol |
| ZnSO₄.7H₂O | Zinc sulphate heptahydrate | White vitriol |
| FeSO₄.7H₂O | Ferrous sulphate heptahydrate | Green vitriol |
| FeSO₄.(NH₄)₂SO₄.6H₂O | Ferrous ammonium sulphate hexahydrate | Mohr’s salt |
There can be some crystals without water of crystallisation, e.g., NaCl crystals.
7. Write the differences between hydrated and anhydrous crystalline solids.
| Hydrated crystalline solid | Anhydrous crystalline solid |
|---|---|
| The crystals that contain a fixed number of water of crystallisation as part of their crystal structure are hydrated crystalline solids. | The crystals that do not possess water of crystallisation are anhydrous crystalline solids. |
| For example: pentahydrated copper sulphate; CuSO₄.5H₂O | For example: sodium nitrate; NaNO₃ |
The constituent particles of a crystal are arranged in a regular repeating manner in a 3D space.
Crystal lattice: A regular three-dimensional arrangement of constituent particles or lattice points in a crystal.
The points occupied by the constituent particles in a crystal are called crystal points or lattice points.
Unit cell: The smallest repeating unit of a crystal lattice that represents the entire lattice when repeated in three dimensions.
8. Explain the process of determination of water of crystallisation.
Ans: The number of water molecules associated with one molecule of solid can be determined by knowing the weight of the anhydrous solid and the amount of water associated with it, and that is the water of crystallisation. This method of determining water of crystallisation is the gravimetric method. In this method, a certain amount of hydrated solid is taken in a clean, dried, preweighed crucible, followed by gradual heating to remove all the water of crystallisation from it. The weight of anhydrous solid is determined after cooling the crucible in a desiccator. The process of heating, cooling, and weighing is repeated until a consistent weight of anhydrous solid is obtained. After knowing the weight of anhydrous solid, associated water, and molecular weight of the compound, we can determine the water of crystallisation of the crystalline compound.
Calculations 🧮
Weight of crucible + hydrated salt = w2
Weight of crucible + anhydrous solid = w3
Weight of hydrated solid = w2 − w1
Weight of anhydrous solid = w3 − w1
Weight of water = w2 − w3
Now, (w3 − w1) g of anhydrous solid is associated with (w2 − w3) g of water molecules. Let the molecular weight of the salt be M; then m g of anhydrous salt is associated with (w2 − w3) × M / (w3 − w1)
Moles of water = (w2 − w3) / 18
So, the number of water molecules associated with one molecule of the salt is = number of water molecules of crystallisation
x = moles of anhydrous salt / moles of water
9. How could the crystal of copper sulphate be prepared?
Ans: To prepare a crystal of copper sulphate, here is the method:
Apparatus: China dish, glass rod, wash bottle, copper sulphate, burner, 250 ml beaker, wire gauze, tripod stand, 25 ml water
Method
- 1. Put the powdered impure sample of copper sulphate in a clean beaker (250 ml).
- 2. Add distilled water and stir the contents gently with a glass rod.
- 3. Add two or three drops of concentrated sulphuric acid to it.
- 4. Heat the solution in the beaker to 60–70°C on a wire gauze.
- 5. Stir it continuously and add more impure copper sulphate until no more of it dissolves.
- 6. Filter the solution and collect the filtrate in a china dish.
- 7. Place the china dish over the wire gauze kept over a tripod stand and heat it gently (do not boil).
- 8. As the solution gets heated, stir it with a glass rod. This helps in uniform evaporation and prevents the formation of a solid crust.
- 9. When the volume of the solution is reduced nearly to half, take out a drop of the solution on the glass rod and cool it by blowing air. Formation of a thin crust indicates that the crystallisation point has been reached.
- 10. Turn off the burner, cover the dish with a watch glass, and keep it undisturbed. As the solution cools down, crystals separate. Slow cooling ensures better crystallisation.
- 11. Decant the mother liquor and wash the crystals with a thin stream of cold water with the help of a wash bottle.
- 12. Dry the crystals by pressing them gently between sheets of blotting paper.
🔥 Heat → 💧 Filter → 🧊 Cool → 💎 Crystals form → 🚿 Wash → 📄 Dry
After this, blue coloured crystals of copper sulphate (CuSO₄ · 5H₂O) will be observed.
With this, one can make a crystal of copper sulphate.
Key Takeaways
| Crystalline solid | Amorphous solid |
|---|---|
| Regular, long-range arrangement of particles | Irregular arrangement; no long-range order |
| Definite geometrical shape | No definite geometrical shape |
| Sharp melting point | Soften/melt over a range of temperatures |
| Generally anisotropic | Generally isotropic |
| Examples → NaCl, quartz | Example → glass |
Anisotropy: Physical properties vary with direction.
Crystallisation: Process of obtaining crystals from a solution, usually by cooling a concentrated/saturated solution.
⭐ Key idea: Hot concentrated solution → slow cooling → crystals form.
Crystal growth: Increase in crystal size as particles arrange themselves regularly in the crystal lattice.
⭐ Easy memory:
Efflorescence → water goes OUT
Hygroscopy → water comes IN
Deliquescence → water comes IN + dissolves
💧 Water of crystallisation is the fixed number of water molecules associated with one formula unit of a crystalline salt.
Hydrated salt: contains water of crystallisation.
Anhydrous salt: does not contain water of crystallisation.
Crystal lattice: Regular three-dimensional arrangement of constituent particles.
Unit cell: Smallest repeating unit of a crystal lattice.