The problems are on the pH scale worksheet. Try them on your own first, then compare with the answers below.
Quick answer: Acids have a pH below 7, alkalis have a pH above 7, and a neutral liquid such as pure water has a pH of exactly 7. These worked answers correct the 20 problems of the grade 8 science worksheet on pH scale, from acid, neutral or alkaline? to the swimming pool: each solution shows the formula, the substitution and the final answer with its unit.
Each answer shows the reasoning first and the result in bold. Check units and the direction in which the pH moves before you look at the number.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS1-2
Problem 1 Acid, neutral or alkaline?
1. pH 3 is acidic, pH 7 is neutral, pH 11 is alkaline and pH 6.5 is acidic (below 7).2. The liquid of pH 6.5 is the closest to 7 without being neutral: a very weak acid.
Problem 2 Litmus colours
1. Blue litmus turns red in acid.2. Red litmus turns blue in an alkali.
3. Neither paper changes colour in pure water, because it is neutral: red stays red and blue stays blue.
Problem 3 Reading universal indicator
1. A (red) is about pH 1 to 2, B (yellow) about pH 5 to 6, C (green) pH 7 and D (purple) about pH 12 to 14.2. Tubes A and B are acidic, C is neutral and D is alkaline.
Problem 4 Ordering by acidity
1. Order from lowest to highest pH: lemon juice, orange juice, tap water, toothpaste mixture, bleach.2. The strongest acid is lemon juice (pH 2.0) and the strongest alkali is bleach (pH 12.5).
Problem 5 Lab safety rules
1. Any three of: wear goggles; never taste chemicals; add acid slowly to water; wash splashes with cold water; tell the teacher at once.2. Mixing releases heat. Adding a little acid to a lot of water keeps the mixture cool, so it does not splash or spit out hot acid.
Problem 6 Finding a dilution factor
1. Factor = 200 mL ÷ 20 mL = 10.2. Factor = 1000 mL ÷ 10 mL = 100.
Problem 7 pH after dilution
1. Factor 10 raises the pH by 1: pH 3.2. Factor 1000 raises the pH by 3: pH 5.
3. It is still acidic, because 5 is below 7. Water alone cannot make a solution alkaline.
Problem 8 Water needed
1. pH must rise by 2, so the factor is 100.2. Final volume = 50 mL × 100 = 5000 mL (5.0 L).
3. Water added = 5000 mL − 50 mL = 4950 mL.
Problem 9 Diluting an alkali
1. Factor 100 changes pH by 2, downwards for an alkali: pH 11.2. The alkali becomes weaker, so the pH falls towards 7, but never goes below 7 from water alone.
Problem 10 Reading a pH graph
1. The graph starts at pH 2.0.2. The pH is 7.0 at 30 mL.
3. Change = 10.5 − 7.0 = 3.5 over 5 mL, so 3.5 ÷ 5 = 0.7 pH units per mL.
Problem 11 Indicator or pH meter?
1. Litmus shows only acid or alkali. Universal indicator gives a rough pH to the nearest unit. A pH meter gives an exact value to 0.1.2. A pH meter or universal indicator, because the gardener needs a number to know how far the pH is from the ideal. Litmus is not precise enough.
Problem 12 Spot the errors
1. The difference is 3 steps, so 10 × 10 × 10 = 1000 times more acidic, not twice.2. Water is neutral, so dilution makes the pH approach 7 but never pass it. The acid cannot reach pH 9.
Problem 13 Comparing two acids
1. The difference is 3 steps: 10³ = 1000 times.2. X would be red and Y would be yellow (orange-yellow for pH 5).
Problem 14 Word equations
1. Hydrochloric acid + magnesium hydroxide → magnesium chloride + water.2. The acid is used up, so the pH moves towards 7.
Problem 15 Soil on a farm
1. She should spread a base such as crushed lime to neutralise the acid soil.2. The pH should rise, from 5.0 towards 6.5.
3. Too much lime would push the pH above the ideal value and make the soil alkaline, which harms the crop.
Problem 16 Dilute to pH 4
1. pH rises by 3, so the factor is 1000.2. Final volume = 25 mL × 1000 = 25 000 mL = 25.0 L.
3. Water = 25 000 mL − 25 mL = 24 975 mL.
Problem 17 Two-stage dilution
1. B has volume 5.0 + 45 = 50 mL, so the factor is 50 ÷ 5.0 = 10.2. C has volume 10 + 90 = 100 mL, so the factor is 100 ÷ 10 = 10.
3. Two tenfold steps raise the pH by 2: pH 2.
4. 10 × 10 = 100 times more dilute.
Problem 18 Rainwater data
1. Leeds (4.6) and Mumbai (3.6) are below 5.6, so they are Leeds and Mumbai.2. The difference is 5.6 − 3.6 = 2 steps, so 10 × 10 = 100 times more acidic.
3. 6.6 is below 7, so it is a weak acid. It is just 10 times less acidic than normal rain.
Problem 19 Planning an antacid test
1. Independent: brand of tablet. Dependent: pH after a set time. Controls: same volume and concentration of acid, same mass of tablet, same stirring and time.2. Measure equal volumes of acid; record the starting pH; add one crushed tablet and stir for 2 minutes; read the pH; repeat for each brand and for repeats.
3. The best brand gives the highest pH closest to 7, without going far above it, averaged over repeats.
Problem 20 The swimming pool
1. pH 8.4 is above 7, so the pool is weakly alkaline.2. She must add a dilute acid.
3. Add it slowly and in small amounts, because a big dose would overshoot and make the water too acidic; wear goggles and gloves and add acid to water, never the reverse.
4. Blue, since the pool is slightly alkaline.
