Gases, Lab Apparatus, Tests & Equations രാസസമവാക്യം
This chapter prepares four gases in the laboratory: hydrogen (Fig 7.1), carbon dioxide (Fig 7.8), oxygen (Fig 7.9) and chlorine (Fig 7.10). To follow those figures you need to know the apparatus by name and how a gas is made and collected. You also need to know why some gases rise out of a jar while others stay in, and the quick tests that tell the gases apart: the “pop” (പോപ്പ് ശബ്ദം), the glowing splinter and lime water (ചുണ്ണാമ്പുവെള്ളം). Finally you will recall from Chapter 5 the types of chemical reactions, and from Chapter 4 how to balance an equation, because the chapter writes every reaction as a balanced equation. Nothing is assumed.
- Chemical symbols and formulas such as H₂, O₂, CO₂ and HCl (page 1 and Chapter 3).
- If you can already name a thistle funnel, say how hydrogen and carbon dioxide are tested, and balance H₂ + O₂ → H₂O, do the sorters and the quiz, then go to page 3.
1. Lab apparatus by name
The figures of this chapter use the same few pieces of glassware again and again. Learn their names and what each one is for; questions often ask “why is this part used?”.
2. How a gas is made in the laboratory
Many gases are made by letting a solid react with a liquid in a closed flask: zinc with dilute hydrochloric acid gives hydrogen; marble (calcium carbonate) with dilute hydrochloric acid gives carbon dioxide (Fig 7.8). The gas has only one way out: through the delivery tube.
3. Gases lighter and heavier than air
Air is a mixture, mostly nitrogen (78 %) and oxygen (21 %). On average a “particle of air” is about 29 times as heavy as a hydrogen atom. A gas whose molecules are lighter than that is less dense than air and rises in it; a gas with heavier molecules is denser than air and sinks. Compare the relative masses: hydrogen H₂ = 2, nitrogen N₂ = 28, oxygen O₂ = 32, carbon dioxide CO₂ = 44, chlorine Cl₂ = 71. That is why hydrogen balloons float away and carbon dioxide can be “poured” over a candle to put it out.
Real gases also mix slowly by diffusion, so in time any open jar loses some gas. The dots are far bigger and fewer than real molecules.
4. Collecting a gas
How a gas is collected depends on two things: is it lighter or heavier than air, and does it dissolve in water?
- Upward displacement of air: the jar stands mouth up and the delivery tube reaches its bottom. A gas denser than air fills the jar from the bottom and pushes the air up and out. Carbon dioxide (Fig 7.8) and chlorine (Fig 7.10) are collected this way.
- Downward displacement of air: the jar is held mouth down. A gas lighter than air fills it from the top and pushes the air down and out: hydrogen.
- Over water: a jar full of water stands upside down in a trough of water. A gas that does not dissolve much in water bubbles up and pushes the water out: hydrogen and oxygen. Chlorine and carbon dioxide dissolve in water, so they are not usually collected this way.
5. Tests for the gases
Most of these gases have no colour and no smell, so they look the same. Simple tests tell them apart:
- Hydrogen: bring a burning splinter (കത്തിച്ച ഈർക്കിൽ കഷണം) to the mouth of the tube. The gas burns with a “pop” sound (പോപ്പ് ശബ്ദം).
- Oxygen: a glowing splinter (one that has just been blown out) bursts into flame; a burning one burns more brightly. Oxygen does not burn itself but helps things burn: it is a supporter of combustion (ജ്വലനസഹായി).
- Carbon dioxide: it turns clear lime water (ചുണ്ണാമ്പുവെള്ളം), a solution of calcium hydroxide Ca(OH)₂, milky. A burning splinter goes out in it.
- Chlorine: it is greenish-yellow with a sharp, choking smell, and it bleaches a moist coloured cloth (page 9). Chlorine is poisonous: only your teacher handles it, in a fume cupboard.
6. Types of chemical reactions (a recap of Chapter 5)
In Chapter 5 you sorted reactions into four types. The textbook of this chapter asks you to use them (page 135):
- Combination (സംയോജന രാസപ്രവർത്തനം): two or more substances join to form one, A + B → AB.
- Decomposition (വിഘടന രാസപ്രവർത്തനം): one compound breaks into two or more substances, AB → A + B, often by heat, light or electricity.
- Displacement (ആദേശ രാസപ്രവർത്തനം): an element pushes another element out of its compound, A + BC → AC + B.
- Double decomposition (ദ്വിവിഘടന രാസപ്രവർത്തനം): two compounds exchange partners, AB + CD → AD + CB, often forming a precipitate (അവക്ഷിപ്തം) or a gas.
7. Balancing equations (a recap of Chapter 4)
Atoms are neither made nor destroyed in a reaction, so a balanced chemical equation (സമീകരിച്ച രാസസമവാക്യം) has the same number of each kind of atom on both sides. You balance it by putting numbers (coefficients) in front of the formulas. You must never change the small numbers inside a formula: changing H₂O to H₂O₂ would make a different substance.
- Apparatus: test tube, boiling tube (for heating), conical and round-bottom flasks, thistle funnel (തിസിൽ ഫണൽ, end below the liquid), delivery tube, gas jar, burner, stand and clamp.
- Lighter than air (hydrogen): mouth down or over water. Denser than air (carbon dioxide, chlorine): mouth up, upward displacement of air.
- Tests: hydrogen – burning splinter, “pop”; oxygen – glowing splinter bursts into flame; carbon dioxide – lime water turns milky; chlorine – greenish-yellow, bleaches moist cloth.
- Reaction types: combination, decomposition, displacement, double decomposition.
- Balance with coefficients in front of formulas; never change a formula.
- Testing hydrogen with a glowing splinter ✗ – that is the oxygen test. Hydrogen: a burning splinter and a “pop”.
- “Carbon dioxide helps a splinter burn” ✗ – it puts the flame out.
- Collecting hydrogen in a jar held mouth up ✗ – it floats out.
- Balancing H₂ + O₂ → H₂O by writing H₂O₂ ✗ – write 2H₂ + O₂ → 2H₂O.