Acids and Alkalis: Lab Safety

Common Acids and Alkalis

• Acids taste sour.
• Alkalis feel slippery; some have a bitter taste.
• Many foods, drinks, and household products contain acids.
• Some foods and household products contain alkalis.
• Hydrochloric, sulphuric, and nitric acids are commonly used in the laboratory.
• Sodium hydroxide, ammonia solution, and calcium hydroxide (lime water) are alkalis commonly used in the laboratory.
• Acids and alkalis can be irritant or corrosive, so handle with care and safety precautions.

Safety Precautions

• Read hazard warning symbols on chemical bottles to identify risks.
• Wear a lab coat, disposable or protective gloves, and safety spectacles.
• Do not mix acids or alkalis with other chemicals without teacher approval.
• Pour waste solutions into suitable waste bottles, not the sink.
• Wash hands thoroughly after handling acids or alkalis.

Emergency Treatments

• In accidents in the lab, remain calm and report to the teacher.
• If acids or alkalis splash on skin, immediately wash the area under slow-running water for at least 10 minutes.
• If in eyes, immediately wash the affected eye(s) using an eyewash bottle for at least 10 minutes.
• If on clothes, carefully remove them, avoiding further contact.

Uses of Acids and Alkalis

• Acids are used for food preservation and stain removal.
• Alkalis are used for making noodles and grease removal.

Distinguishing Acids and Alkalis

• Some plants contain pigments that change color in acidic or alkaline solutions, acting as acid-alkali indicators.
• Litmus is a common acid-alkali indicator with blue and red forms.
• Litmus is available in solution or as litmus paper.

Litmus Paper Color Changes

• Blue litmus turns red in acidic solutions, remains blue in neutral and alkaline solutions.
• Red litmus turns blue in alkaline solutions, remains red in acidic and neutral solutions.

pH Scale

• pH scale describes the acidity or alkalinity of a substance.
• Acidic substances have a pH lower than 7.
• Neutral substances have a pH of 7.
• Alkaline substances have a pH higher than 7.
• Universal indicator and pH paper are used to measure pH values.
• A pH meter is an electronic instrument for measuring pH values.

Universal Indicator vs pH Meter

• Universal indicator is convenient (no calibration needed) using less solution
• Universal indicator is not suitable for dark-colored solutions, cannot be reused and has subjective results
• pH meter works for any solution, can be reused and provides small reading error.
• pH meter needs calibration and more solution.

Neutralization

• Adding an alkali to an acid increases the pH; adding an acid to an alkali decreases the pH.
• When pH reaches 7, the solution is neutral, and the acid and alkali neutralize each other.
• Neutralization forms salt and water, releasing thermal energy.
• The general equation for neutralization is: acid + alkali -> salt + water.
• The type of salt formed depends on the acid and alkali used.

Steps in Writing Neutralization Equations

• Write down the reactants.
• The salt name has two parts: the first from the alkali, the second from the acid.
• Add "water" as the other product.
• Reactants and products mass is conserved during neutralization.

Applications of Neutralization

• Antacids (alkaline) neutralize excess stomach acid to relieve pain.
• Toothpaste (slightly alkaline) neutralizes mouth acids to prevent tooth decay.
• Chemical waste (acids or alkalis) is neutralized before discharge.
• Soap solutions (alkali) neutralize acidic bee/mosquito/ant stings.
• Vinegar (acid) helps neutralize alkaline wasp stings.
• Alkaline lime is added to acidic soil to raise pH.
• Sulphur converted to sulphuric acid lowers the soil pH.

Corrosive Nature of Acids

• Dilute acids react with some metals to produce hydrogen, corroding the metal.
• General equation: metal + acid -> salt + hydrogen.
• The type of salt produced depends on the acid and metal:
  • zinc + hydrochloric acid -> zinc chloride + hydrogen
  • calcium + sulphuric acid -> calcium sulphate + hydrogen
• Dilute acids corrode building materials containing calcium carbonate like marble/limestone.
• Dilute acids reacting with calcium carbonate produce carbon dioxide.
• General equation: carbonate + acid -> salt + water + carbon dioxide.
• Salt type depends on the acid and carbonate:
  • calcium carbonate + hydrochloric acid -> calcium chloride + water + carbon dioxide
  • magnesium + sulphuric acid -> magnesium sulphate + water + carbon dioxide

Acid Rain

• Natural rainwater pH is about 5.6; below 5.6 is acid rain.
• Burning fossil fuels releases sulphur dioxide and nitrogen oxides.
• Acidic gases react with oxygen and water, increasing rainwater acidity.

Effects of Acid Rain

• Acid rain damages plant leaves, reducing photosynthesis.
• Toxic minerals released into soil harm plant growth.
• Acid rain washes toxic minerals into water bodies, killing aquatic life.
• Acid rain corrodes buildings/statues with calcium carbonate.
• Acid rain corrodes metal objects.

Hazards of Acids and Alkalis

• Concentrated acids and alkalis are highly corrosive, handle with extra care.
• Mixing cleansing products can be dangerous, releasing toxic substances and thermal energy.

Safety Rules for Cleansing Products

• Keep the room well-ventilated.
• Wear gloves, a mask, and protective clothing.
• Wash hands thoroughly after use.
• After an accident, remain calm and provide proper treatment.
• If splashed on the body, wash with a lot of slow-running water.
• If a harmful gas is inhaled, open windows and move the injured person to fresh air.
• If the injury is serious, take the injured person to a hospital ASAP.