Acids, Bases & pH — Practice · Grade 8 Science

Chapter: Chemistry

What this practice covers

By the end of Acids, Bases & pH you should be able to:

11 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: acid, base, pH scale, indicator, neutralization, model, evidence, system boundary, assumption.

Problem 1

A substance with a pH of 10 is…
  1. strongly acidic
  2. neutral
  3. basic
  4. impossible, since pH can't exceed 7
Answer and explanation

basic

A pH above 7 is basic; 10 is well above 7.

Problem 2

Pure water has a pH of ___.
Answer and explanation

7

Pure water sits exactly at the neutral point, pH 7.

Problem 3

Which everyday substance is most likely a strong base?
  1. orange juice (pH ≈ 3.5)
  2. black coffee (pH ≈ 5)
  3. drain cleaner (pH ≈ 13)
  4. milk (pH ≈ 6.5)
Answer and explanation

drain cleaner (pH ≈ 13)

A pH near 13 is far above 7, making drain cleaner a strong base.

Problem 4

Explain how an indicator like litmus paper is used to test whether a solution is acidic or basic.
Answer and explanation

You dip the litmus paper into the solution; if it turns red, the solution is acidic, and if it turns blue, the solution is basic. The color change tells you the general pH range without needing a meter.

Problem 5

An acidic solution has a pH of 4. After adding a base, its pH rises by 3. What is the new pH?
Answer and explanation

7

New pH = 4 + 3 = 7, which is neutral.

Problem 6

Mixing an acid and a base together in a neutralization reaction produces water and a…
  1. gas only
  2. salt
  3. new acid
  4. new base
Answer and explanation

salt

Neutralization reactions between an acid and a base produce water plus a salt.

Problem 7

Compare the hydrogen-ion activity in solutions of pH 3 and pH 5. Explain the reasoning in your own words.
Answer and explanation

The pH 3 solution has about 100 times the hydrogen-ion activity of pH 5 under comparable conditions.

The pH difference is 5−3=2.

Each step is a factor of 10, so the factor is 10×10=100.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 8

Compare hydrogen-ion activity at pH 5 and pH 6. Show the reasoning, not just the conclusion.
Hints

A lower pH must correspond to greater, not smaller, hydrogen-ion activity.

Answer and explanation

The pH 5 sample has 10 times the activity, because the difference is one logarithmic unit.

Problem 9

Can two solutions with equal pH always be treated as equally safe? Explain.
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

No. Toxicity, concentration, total amount, chemical identity and exposure also matter; pH is only one property.

Problem 10

An indicator color estimates a range with uncertaintyAn indicator color estimates a range with uncertaintyReading 1Reading 2Reading 3Key: pH 5–6Key: pH 5–6Unclear matchSupplied indicator key only: color means a range, not an exact decimal.Compare fresh samples, consistent lighting and the correct reference chart.
Two illustrative test colors match the supplied pH 5–6 key while a third reading is ambiguous. These colors are a specific provided key, not a universal rule for every indicator. An indicator’s transition range, lighting, amount and sample color can limit the estimate. No taste, skin test or mixing of unknown chemicals is intended.
For this unit’s illustrated case, what pH claim is supported, and why would reporting 5.34 be unjustified?
Answer and explanation

A range around pH 5–6 is supported by the two clear matches, with the ambiguous result noted. The color method does not supply precision to two decimal places.

A range expresses what the method can resolve. Repeat a consistent approved measurement and compare the actual key. Record the uncertain match instead of forcing every reading into a precise number.

A range around pH 5–6 is supported by the two clear matches, with the ambiguous result noted. The color method does not supply precision to two decimal places.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 11

A supplied indicator key gives a pH range 8–9 for a sample. Compare it qualitatively with a pH 6 sample and explain why the range does not support an exact pH 8.37 claim.
Answer and explanation

The 8–9 sample is basic and the pH 6 sample acidic under the stated common room-temperature aqueous convention. Indicator color supplies a range, not that exact decimal.

Use the supplied key and conditions.

Respect measurement resolution and the logarithmic scale.

The 8–9 sample is basic and the pH 6 sample acidic under the stated common room-temperature aqueous convention. Indicator color supplies a range, not that exact decimal.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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