Class 9 Chemistry Notes – Chapter 13

Laboratory and Practical Skills

Textbook Exercise Solutions

1. Tick (✓) the Correct Answer

(i) Safety in the chemistry laboratory is:

Answer: (d) A shared responsibility

(ii) Accidents often result from:

Answer: (d) All of the above

(iii) The label “Warning” on a chemical bottle signifies:

Answer: (c) That the user should be careful when using the chemical.

(iv) The label “Corrosive” on a chemical bottle indicates:

Answer: (c) That its contact destroys living tissue.

(v) Example of a highly toxic chemical:

Answer: (c) Potassium cyanide

(vi) Example of a self-reactive chemical:

Answer: (c) Picric acid

(vii) When diluting an acid with water:

Answer: (c) Always add acid to water.

(viii) What should you do in case of a fire drill in the laboratory?

Answer: (c) Close gas valves and turn off all equipment.

2. Questions for Short Answers

(i) Name some corrosive chemicals.

Answer: Concentrated sulfuric acid (H₂SO₄), concentrated hydrochloric acid (HCl), concentrated nitric acid (HNO₃), sodium hydroxide (NaOH), and potassium hydroxide (KOH) are some common corrosive chemicals.

(ii) What type of safety precautions are adopted to avoid damage due to explosive chemicals?

Answer: Store explosive chemicals away from direct heat, sparks, open flames, and sunlight. Avoid mechanical shock and friction. Handle them in extremely small quantities under a safety shield or in a fume hood, and wear appropriate personal protective equipment (PPE), including safety goggles and gloves.

(iii) What type of damage can reactive chemicals cause?

Answer: Reactive chemicals can undergo violent chemical changes, causing sudden explosions, fires, the rapid release of toxic gases, thermal or chemical burns to human skin, and structural damage to the laboratory.

(iv) Indicate two safety instructions required to avoid radiation hazards.

Answer:

1. Maintain proper shielding and distance when handling radioactive sources, and use suitable remote handling tools.

2. Limit exposure time and wear a radiation-monitoring badge (dosimeter) where required.

(v) Which chemicals can cause suffocation?

Answer: Asphyxiant or hazardous gases include carbon monoxide (CO), chlorine (Cl₂), sulfur dioxide (SO₂), hydrogen sulfide (H₂S), and ammonia (NH₃). Carbon dioxide (CO₂) and nitrogen (N₂) can also cause suffocation by displacing oxygen in confined spaces.

(vi) Why are signs and symbols posted in laboratories and on chemical bottles?

Answer: Safety signs and hazard pictograms provide immediate visual warnings about chemical hazards, such as flammability, toxicity, corrosiveness, and radioactivity. They guide laboratory users in handling chemicals safely and help prevent accidents.

(vii) How should fires caused by chemicals be handled?

Answer: If it is safe to do so, turn off the gas supply and electricity. Never use water on electrical fires or fires involving reactive metals. Use the appropriate fire extinguisher, such as CO₂ or dry chemical powder, according to the type of fire. Evacuate the area and alert the responsible personnel if the fire is uncontrolled.

(viii) Why are emergency drills important for facing emergency situations?

Answer: Emergency drills train students and laboratory personnel to respond calmly and quickly during emergencies. They ensure familiarity with evacuation procedures and the locations of safety equipment, reducing panic and the risk of injuries.

3. Constructed Response Questions

(i) How will you handle an emergency situation caused by fire due to short-circuiting?

Answer: Immediately isolate the main electrical power supply if it is safe to do so. Do not use water to extinguish an electrical fire. Use a suitable CO₂ or dry chemical fire extinguisher only if trained and it is safe to do so. Alert laboratory occupants and initiate safe evacuation procedures.

(ii) What type of reactions should be carried out in a fume cupboard?

Answer: Reactions that release toxic, irritating, choking, or flammable gases, vapours, or fine particles should be carried out in a properly functioning fume cupboard. Examples include reactions involving chlorine (Cl₂), hydrogen sulfide (H₂S), and nitrogen dioxide (NO₂), as well as procedures involving concentrated acids that release hazardous fumes.

(iii) Put forward at least two suggestions to improve safety in the laboratory.

Answer:

1. Install fully functional safety showers, eyewash stations, and appropriate fire extinguishers in accessible locations.

2. Conduct mandatory safety-orientation training and routine equipment inspections before allowing experimental work.

(iv) Can you identify warning symbols posted for radiation and asphyxiant chemicals?

Answer:

• Radiation Symbol: A trefoil symbol consisting of three blades surrounding a central circle, commonly displayed on a yellow background.

• Gas Hazard Symbol: A gas-cylinder pictogram inside a red diamond frame, used to indicate gases under pressure. This symbol does not, by itself, identify every asphyxiant gas.

(v) Why can sudden shock cause some chemicals to explode?

Answer: Certain shock-sensitive chemicals, such as some peroxides, fulminates, and improperly stored picric acid, can undergo rapid decomposition when exposed to impact or friction. This can trigger a rapid, exothermic reaction that releases heat and gases, potentially causing an explosion.

4. Descriptive Questions

(i) Explain the hazards caused by explosive and toxic chemicals.

Answer:

Explosive Hazards: Chemicals such as certain peroxides and nitrated organic compounds may undergo rapid decomposition when triggered by heat, shock, or friction. This can cause blast injuries, flying debris, and intense fires.

Toxic Hazards: Chemicals such as potassium cyanide and certain heavy-metal salts can act as poisons. They may enter the body through inhalation, ingestion, or skin absorption, causing severe organ damage, systemic toxicity, respiratory failure, or even death.

(ii) Write down five common safety instructions used to avoid different types of hazards.

Answer:

1. Always wear appropriate personal protective equipment (PPE), including safety goggles, a laboratory coat, enclosed shoes, and suitable protective gloves.

2. Never eat, drink, taste chemicals, or directly inhale them in the laboratory.

3. Always add concentrated acid slowly to water, never water to concentrated acid.

4. Know the locations and proper use of safety showers, eyewash stations, fire extinguishers, and emergency exits.

5. Perform reactions that release hazardous fumes or toxic gases inside a properly functioning fume hood.

(iii) Explain the importance of warning signs and symbols in preventing laboratory accidents.

Answer: Warning signs and hazard symbols communicate important information about chemical hazards quickly and clearly. They indicate whether a chemical is flammable, corrosive, toxic, radioactive, or explosive. This information helps laboratory users select appropriate protective equipment, follow safe storage procedures, and handle chemicals correctly, thereby reducing the risk of accidents and injuries.

(iv) Name some toxic chemicals. Describe the effects of toxic gas spreading in a laboratory.

Answer: Toxic chemicals include chlorine gas (Cl₂), hydrogen sulfide (H₂S), carbon monoxide (CO), and potassium cyanide (KCN).

The spread of toxic gas in a laboratory can cause eye and respiratory irritation, coughing, choking, dizziness, loss of consciousness, lung injury, and potentially death. If a toxic gas leak occurs, occupants should leave the affected area immediately, warn others, and contact trained emergency responders. Untrained persons should not attempt to stop the leak or enter the contaminated area.

(v) A student has spilled a corrosive and explosive chemical due to an accident. What emergency measures will you take to tackle the situation?

Answer:

1. Alert everyone nearby and immediately move away from the spill area.

2. Inform the laboratory instructor or safety officer and follow the laboratory’s emergency procedures.

3. If the chemical contacts the skin or eyes, immediately use the safety shower or eyewash station and flush with running water for at least 15 minutes, unless the chemical’s specific safety instructions require a different procedure.

4. Keep people away and eliminate ignition sources only if this can be done safely without approaching the spill.

5. Do not touch, mix, neutralize, or attempt to clean up the chemical yourself. Some chemicals react dangerously with water or other substances.

6. Leave ventilation systems and windows alone unless instructed by trained emergency personnel, because changing airflow may spread hazardous vapours.

7. Allow only trained personnel with appropriate protective equipment and spill-control materials to manage the spill.

5. Investigative Question

(i) A few decades ago, a tanker carrying poisonous chlorine gas leaked, and the gas spread over a large area in Lahore. The accident killed a few people as well as animals. Give some concrete proposals to avoid such an accident in the future.

Answer:

1. Strict Transport Standards: Transport hazardous compressed gases in properly designed, pressure-tested, corrosion-resistant tankers fitted with suitable safety valves and leak-detection systems.

2. Bypassing Urban Centres: Where possible, route gas tankers through designated industrial transport corridors and bypass routes to avoid densely populated areas.

3. Emergency Response Equipment: Equip transport vehicles with suitable emergency leak-control equipment, gas detectors, and GPS tracking systems. Only trained hazardous-material response teams should attempt to control chlorine leaks.

4. Community Awareness and Emergency Planning: Conduct public safety awareness programmes and establish emergency response plans for communities located near hazardous-material transport routes.

5. Regular Inspection and Training: Inspect tankers, valves, pipelines, and safety systems regularly. Train drivers and emergency personnel in hazardous-gas transport, leak recognition, evacuation, and emergency communication procedures.

6. Effective Warning and Evacuation Systems: Establish rapid alert systems so nearby communities can receive timely warnings and follow instructions from emergency authorities during a chlorine leak.

Notes by Sir Dilawar

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