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Chemistry · Fundamentals of chemistry · Grade 7

The chemistry lab and safety

Why must a test tube be pointed away from people when heated?

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The situation

The first chemistry lesson of the new school year. In the chemistry room there are stands, flasks, test tubes, a spirit lamp, a gas burner and a balance, and on the wall hang signs with red borders. What is each piece of apparatus for? Why must you never point the mouth of a heated test tube at people, and why is it forbidden to blow out a spirit lamp? And when, and by whom, were many of these instruments invented?

Topic

The chemistry lab: equipment, safety and the history of chemistry. A school chemistry lab: spirit lamp, stand, gas burner and safety signs — from Jabir ibn Hayyan and al-Razi's alembic to the modern laboratory

Key ideas

  • Laboratory equipment
  • Safety rules
  • History of chemistry

Formulas

Vliquid≤13Vtube

The liquid being heated should not exceed one third of the volume of the test tube (≈ 7 mL in a test tube of ≈ 22 mL).

m(salt)=ω⋅m(solution)=0.05⋅50g=2.5g

The mass of salt to weigh out to prepare 50 g of a 5 % solution.

Q=cmΔt

The heat needed to warm the water; during boiling the temperature stops rising at 100 °C.

C2H5OH+3O2→2CO2+3H2O

Ethanol burns in a spirit lamp; the flame goes out because the cap cuts off the oxygen.

Theory

The science of chemistry and the chemistry room

Chemistry is the science that studies substances, their composition, structure and properties, and how they change into one another. Chemical knowledge is needed in the production of medicines, fertilisers, fuel, plastics, metals and food: in Uzbekistan nitrogen fertilisers are made at Navoiyazot, Ferganaazot and Maxam-Chirchiq, copper at the Almalyk Mining and Metallurgical Complex, soda at the Kungrad Soda Plant and polyethylene at the Shurtan Gas Chemical Complex. A chemist experiments, which is why the chemistry room is specially equipped: a fume cupboard, water and drainage, a fire extinguisher and a first-aid kit.

Laboratory equipment

Glassware: test tube (small-scale experiments), beaker (preparing solutions), conical and round-bottomed flasks (mixing, heating, distillation), measuring cylinder (measuring volume — read at the bottom of the meniscus), funnel (pouring and filtering). Porcelain: evaporating dish (evaporation), mortar (grinding). Supports: stand with ring and clamp, test-tube rack, test-tube holder, tripod and gauze. Heating devices: spirit lamp, gas burner, electric heater. The balance measures mass: the substance is placed on paper or in a container, and the mass of the container is set to zero with the “Tare” button. A clean, washed vessel has a smooth film of water on its wall, not droplets.

Safety rules and first aid

In the chemistry room, wear safety goggles and a lab coat; substances are never tasted, and smells are checked by wafting the vapour towards you with your hand. Add acid to water, never the reverse: concentrated sulfuric acid is denser than water (ρ ≈ 1.84 g/mL); poured into water, it sinks and the heat released spreads through all the water; if water is poured into acid, the light water stays on top of the acid, suddenly boils and splashes the acid. The GHS symbols on reagent containers (9 pictograms with a red border) show the hazard: flammable, oxidising, corrosive, toxic … First aid: if acid or alkali gets on the skin, rinse immediately with plenty of running water; if it gets in the eyes, rinse for 15 minutes and see a doctor; for a burn, use cold water; tell the teacher at once about any accident. If a fire starts: if spilt alcohol or petrol on the bench catches fire, put out the flame by sprinkling sand or covering it with a fire blanket (never use water on petrol); if an electrical appliance catches fire, first cut off the current, then use the extinguisher.

Rules for heating

The cap of a spirit lamp is removed and the lamp is lit with a match; one lamp is never lit from another, and it is not blown out — it is covered with the cap: with no oxygen getting in, the flame goes out. The hottest part of the flame is its upper part; the inner part around the wick is cooler. In a gas burner, if air is let in through the hole at the bottom, the flame is blue and hot (hottest just above the tip of the inner cone); with little air it is yellow, bright, sooty and cooler. Heating a test tube: the liquid must not exceed 1/3 of the volume; the tube is held with a holder near its mouth, tilted at 30–60°, and its mouth is pointed away from yourself and others; first the whole tube is warmed gently, then the upper part of the liquid. Otherwise liquid overheated at the bottom suddenly boils and shoots out. An electric heater has no flame — convenient for heating flammable liquids.

From the history of chemistry

Operations such as distillation, crystallisation and sublimation were mastered by scholars of the East in the 9th–10th centuries: the works attributed to Jabir ibn Hayyan, the book on perfumes and distillation by al-Kindi, the “Book of Secrets” by Abu Bakr al-Razi (classification of substances, the still, retort, crucible). The words “alembic”, “alchemy” and “alcohol” passed into European languages from Arabic. Al-Biruni measured the relative density of metals and precious stones accurately; Ibn Sina described hundreds of medicinal substances in the “Canon of Medicine” and showed that metals cannot be turned into gold; al-Khwarizmi and Ahmad al-Farghani were famous mathematicians and astronomers. In the 20th century in Uzbekistan S. Yu. Yunusov (plant alkaloids) and O. S. Sodiqov (bioorganic chemistry) created scientific schools.

About the model

The heating power delivered to the test tube is approximate (≈ 25 W at the top of a spirit lamp, ≈ 60 W for a gas burner), and heat loss to the air is simplified; heating runs 4 times faster than real time. “Shooting out” is a qualitative model of boiling in a filled or vertically held test tube. The alembic scene is sped up 30 times and calculated with ≈ 150 W of useful heat and 90 % condensation. The acid dilution demonstration is qualitative (temperatures are not shown). The GHS symbols of the reagents follow the European Union (CLP) classification and manufacturers' safety data sheets. The exact temperature of a spirit lamp flame is given differently in different sources, so no figure is given.

Did you know?

  • The word “alembic” comes from the Arabic “al-anbiq” (the lid of a distillation apparatus); Abu Bakr al-Razi described apparatus such as the still, retort and crucible in the “Book of Secrets”.
  • GHS is a single system for classifying and labelling chemicals developed by the United Nations: it has 9 pictograms, all drawn as a black symbol on a white diamond with a red border.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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