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Acids and Bases

Acids and Bases. pH Scale Basics. The pH scale is used to classify acids and bases (also known as “alkali”) of different strengths. The letters pH stand for the “potential of hydrogen”. The pH values are determined by the concentration of hydrogen ions (H+) in the solution.

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Acids and Bases

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  1. Acids and Bases

  2. pH Scale Basics • The pH scale is used to classifyacids and bases (also known as “alkali”)of different strengths. • The letters pH stand for the “potential of hydrogen”. • The pH values are determined by the concentration of hydrogen ions (H+) in the solution. • The pH scale has values that range from zero to fourteen. • The pH values of acids are at one end of the scale, and those of bases at the other. • A neutral solution such as pure water, which is neither acidic nor basic, has a pH value of seven. • An acid has a pH value less than seven • A base (alkaline) has a pH value greater than seven. increasingly acidic increasingly basic 0 14

  3. Acids (H+) • pH < 7 • Acidsare ionic compounds (a compound with a positive or negative charge) that break apart in water to form a positively charged hydrogen ion (H+). • The higher the concentration of H+ ions present in a solution, the stronger the acid • React with metals such as, magnesium and zinc, and release hydrogen gas • Strong acids are good conductors of electricity; weak acids are not. • Have a sour taste • Acids turn blue litmus paper red; the color of cabbage juice changes from blue to red/pink

  4. Bases (OH-) • pH > 7 • Bases are ionic compounds that break apart to form a negatively charged hydroxide ion (OH-) in water. They are also known as alkali. • Strong bases are good conductors of electricity; weak bases are not. • Basic solutions have a bitter taste • Basic solutions feel slippery to the touch • Bases turn red litmus paper blue; Bases change the color of cabbage juice from blue to yellow/green

  5. Neutral • pH =7 • Neither an acid nor a base • The concentration of H+ ions in the solution equals the concentration of OH- ions • Pure water is a neutral substance

  6. ACID-BASE Indicators Yellow Litmus Paper

  7. ACID-BASE Indicators Cabbage Juice Indicator Key

  8. Problem: Can I identify the four mystery liquids, A, B, C and D? Predict: Observe the actual pH value for each of the four liquids. Use the actual pH value that has been provided to determine whether the liquid is an acid, a base or neutral. Predict the color change that you would expect to see when each of the liquids below is tested with the given indicator.

  9. SAFETY: Do NOT taste or smell the test liquids. Ingesting or inhaling the chemicals is hazardous to your health. Do NOT mix the liquids. Wash hands thoroughly after handling the liquids.

  10. Procedure: • Observe each liquid in the 4 containers. Do not smell, taste or touch the liquid. • Record your observations in the chart below. • Carefully open container A. Dip a pH indicator strip into the liquid. Remove the pH indicator strip and close the container. Record the color of the pH indicator strip. Repeat for containers B, C and D using a new pH indicator strip each time. • Use a clean pipette to fill a test tube with 10 drops of cabbage juice. • Use a clean pipette to extract liquid from container A. • Add up to 20 drops of the liquid to the cabbage juice in the test tube until you observe a change in color. Do not allow the pipette to touch the liquid. Record your observations. • Repeat steps 4-6 for liquids B, C, and D.

  11. Conclude Analyze the data you collected. Given the choices: milk, vinegar, ammonia and distilled water, identify the four unknown liquids, A, B, C and D. Justify your claims with evidence and reasoning.

  12. Thinking Deeper • Which of the four liquids would be the best conductor of electricity? Why? • 2. What do you think would happen if you mixed a strong acid and a strong base? Why?

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