C. Chemistry: Chemical Reactions · Chemical Reactions

Acids, Bases and the pH Scale

C3.7 C2.6 C2.1 A1.1 A1.2 A1.6 A1.12About 75 minutes

Learning goals

I can explain acids and bases in terms of the H⁺ and OH⁻ ions they produce in water.
I can use the pH scale to classify solutions as acidic, basic or neutral.
I can explain that each pH unit is a factor of 10 and compare the acidity of two solutions.
I can choose an indicator or instrument and plan and carry out an inquiry to classify household substances.

Words to know

base
A compound that produces hydroxide ions (OH⁻) in water; its solutions have a pH above 7.

alkaline
Another word for basic.

pH scale
A scale, usually 0 to 14, that shows how acidic or basic a solution is; 7 is neutral.

neutral
Neither acidic nor basic; pH 7.

indicator
A substance that changes colour depending on pH.

universal indicator
A mixture of indicators with a different colour for each pH value.

pH meter
An electronic instrument that measures pH precisely.

Sour, slippery and in between

A squeeze of lemon on fish, a fizzing antacid, the sting of shampoo in your eyes, a garden that grows blueberries but not lilacs: all of these depend on whether something is acidic, basic or neutral. Chemists, doctors, farmers, pool operators and water treatment workers measure this every day with the pH scale.

In this lesson you will review acids and bases from Grade 9, add a deeper explanation of what makes a solution acidic or basic, and plan and conduct an inquiry to classify common substances (expectations C2.6 and C3.7).

Remember from Grade 9

In Grade 9 you learned that acids taste sour and turn blue litmus red, bases feel slippery and turn red litmus blue, the pH scale runs from about 0 to 14 with 7 neutral, and each step of 1 is a factor of 10. You made a red cabbage indicator and learned never to mix cleaning products. Now you will explain these properties with ions.

What makes an acid or a base?

In the acid-naming lesson you learned that an acid produces hydrogen ions, H⁺, when it dissolves in water:

HCl(aq) → H⁺(aq) + Cl⁻(aq)

A base produces hydroxide ions, OH⁻, when it dissolves in water. Many bases are ionic compounds that contain hydroxide:

NaOH(aq) → Na⁺(aq) + OH⁻(aq)

Some compounds without hydroxide in their formula still act as bases, because they react with water to make OH⁻ ions. Examples are ammonia (NH₃, in window cleaners), sodium carbonate (washing soda) and sodium hydrogen carbonate (baking soda).

More H⁺ ions than OH⁻ ions: the solution is acidic.
More OH⁻ ions than H⁺ ions: the solution is basic (also called alkaline).
Equal amounts: the solution is neutral, like pure water.

Property	Acids	Bases
Ion produced in water	H⁺ (hydrogen ion)	OH⁻ (hydroxide ion)
pH	less than 7	greater than 7
Litmus paper	blue litmus turns red	red litmus turns blue
Taste and feel (never test this!)	sour	bitter, slippery
Reaction with active metals (e.g. Mg, Zn)	produce hydrogen gas	usually no reaction
Reaction with carbonates (e.g. baking soda)	produce carbon dioxide gas	no reaction
Conduct electricity in solution	yes (ions present)	yes (ions present)
Examples	HCl(aq), H₂SO₄(aq), CH₃COOH(aq) (vinegar), citric acid	NaOH, Ca(OH)₂, Mg(OH)₂, NH₃(aq), NaHCO₃

The pH scale

The pH scale is a number scale, usually from 0 to 14, that tells how acidic or basic a solution is.

pH less than 7: acidic. The lower the pH, the more acidic. A solution with a pH of 1 is highly acidic.
pH 7: neutral.
pH greater than 7: basic. The higher the pH, the more basic.

The pH scale is based on the concentration of hydrogen ions. Each step of 1 pH unit means a 10 times change in H⁺ concentration. So:

pH 3 is 10 times more acidic than pH 4,
pH 3 is 100 times (10 × 10) more acidic than pH 5,
pH 2 is 1000 times more acidic than pH 5.

A small change in pH number is a big change in acidity. That is why a lake whose pH drops from 6.5 to 5.5 has become 10 times more acidic, enough to harm many fish and frogs.

The pH scale with typical values for common substances. Pure water, at pH 7, is neutral. Values are approximate, because products and samples vary. Clean rain is slightly acidic (about 5.6) because carbon dioxide from the air dissolves in it.

From pH 3 to pH 5 the hydrogen ion concentration is divided by 10 twice, so pH 5 is 100 times less acidic than pH 3.

Measuring pH: choosing the right tool

Expectation A1.2 asks you to select appropriate instruments for an inquiry. For acids and bases you can choose from:

Litmus paper: quick, but only tells you acid or base (not the pH number).
Chemical indicators: substances that change colour over a certain pH range. Different indicators change at different pH values, so chemists pick the one that suits their question.
Universal indicator and pH paper (test strips): a mixture of indicators that gives a different colour for each pH, so you can estimate the pH number. Pool and aquarium test strips work this way.
pH meter: an electronic probe that gives a precise number, such as 6.84. Used in labs, water treatment plants and farms.

Colour changes of four common lab indicators across the pH scale. The colours themselves are the information here. Each indicator changes colour over a different pH range.

Worked example

Example 1: How many times more acidic?

Stomach acid has a pH of about 2. A cup of black coffee has a pH of about 5. How many times more acidic is stomach acid?

Difference in pH: 5 − 2 = 3 units.
Each unit is a factor of 10, so the factor is 10 × 10 × 10 = 10³.

Stomach acid is about 1000 times more acidic than coffee.

Worked example

Example 2: Choose an indicator

You need to tell whether a sample has a pH of about 3 or about 5. Which indicator from the chart would work best? What colours would you see?

Litmus would be red for both (both are below about 5), so it can't tell them apart.
Bromothymol blue would be yellow for both (both below 6). Phenolphthalein would be colourless for both.
Methyl orange changes between pH 3.1 and 4.4: it is red at pH 3 and yellow at pH 5.

Use methyl orange: red means about pH 3, yellow means about pH 5. A good indicator changes colour between the pH values you want to tell apart.

Worked example

Example 3: Interpret indicator results

A colourless solution turns red litmus blue, turns bromothymol blue to blue and stays colourless with phenolphthalein. Estimate its pH.

Red litmus turns blue: the solution is basic (pH above about 8).
Bromothymol blue is blue: pH above 7.6. Consistent.
Phenolphthalein stays colourless: pH below 8.2.
Putting it together: litmus says above about 8, phenolphthalein says below 8.2.

The pH is about 8: weakly basic, like a baking soda solution. Using several indicators narrows down the pH.

Hands-on activity

Inquiry: classify household substances

Question: Which common household substances are acidic, basic or neutral, and roughly what is their pH?

You need: a grown-up, eye protection, pH test strips (sold for pools, aquariums or gardening) or red cabbage indicator (as you made in Grade 9: a grown-up pours hot water over chopped red cabbage, and you strain it after 10 minutes), clear cups, labels, a spoon, and these test substances: lemon juice, white vinegar, a clear pop, milk, tap water, baking soda (1/2 teaspoon in 1/4 cup of water), an antacid tablet crushed in 1/4 cup of water, a little dish soap in water, and a little toothpaste stirred into water.

Safety: eye protection on. Do not test bleach, drain cleaner, oven cleaner, ammonia or toilet-bowl cleaner: they are corrosive and can react dangerously. Never mix products. Do not drink or taste anything.

Plan it first (A1.1, A1.2):

Write a prediction for each substance (acidic, basic or neutral) with a reason.
Choose your instrument (strips or cabbage indicator) and explain why.
Identify your control (tap water) and your controlled variables (same amount of indicator, same amount of sample, clean spoon each time).
Make a data table: Substance, Prediction, Colour observed, Estimated pH, Acidic/basic/neutral.

Do it: test each substance (dip a strip, or add 1 tablespoon of sample to 2 tablespoons of cabbage indicator). Compare with the colour chart on the strip package or the cabbage chart. Repeat any surprising result once.

What to look for: lemon juice and vinegar (about pH 2 to 3) and pop (about pH 3) are acidic; milk is close to neutral (about 6.5); tap water is about 7 (often a bit higher in Ontario); baking soda solution (about 8), toothpaste (often about 7 to 9), the antacid (about 9 to 10) and soap solution (about 8 to 10) are basic. Exact values depend on the brand. In your conclusion, compare with your predictions and name a source of uncertainty (for example, judging colours by eye).

Just for fun

What did the pH 7 solution say when the acids and bases asked it to pick a side?

"Sorry, I'm staying neutral on this one."

Your turn

What ion do acids produce in water? What ion do bases produce?

Classify each pH as acidic, basic or neutral: 1, 6.5, 7, 9, 13.

How many times more acidic is pH 4 than pH 6?

Why can't litmus paper tell you whether a solution has a pH of 2 or 4?

Write the equation for sodium hydroxide dissolving in water.

Phenolphthalein is added to a solution and stays colourless. Can you conclude the solution is acidic? Explain.

Name two instruments for measuring pH and say when you would choose each.

More practice: warm-up

What is the pH of pure water?

Is a substance with pH 11 acidic or basic?

What colour does blue litmus turn in an acid?

Name a base found in a home.

Which is more acidic, pH 2 or pH 5?

More practice: on your own

A solution's pH changes from 8 to 11. Is it more or less basic now, and by what factor?

Bromothymol blue turns green in a sample. What can you say about its pH?

Explain why both acids and bases conduct electricity in solution, but pure water hardly does.

Normal rain has a pH of about 5.6. A sample of rain from near a smelter has a pH of 3.6. How many times more acidic is it?

A student tests three samples with red cabbage indicator: A turns pink, B stays purple, C turns green. Classify each. (Use your Grade 9 cabbage colour chart: red-pink acidic, purple about neutral, blue-green to green basic.)

Challenge

You have four unlabelled solutions with pH values of 2, 6, 9 and 12, and only litmus, bromothymol blue and phenolphthalein. How far can you get in identifying them? Use the indicator chart.

A pool should have a pH of about 7.2 to 7.8. The test shows 8.4. Is the water too acidic or too basic? Should the operator add an acid or a base to fix it?

A student says, "A solution with pH 0 has no acid in it, because zero means nothing." Explain the error.

Check your understanding

Quiz

A solution with a pH of 1 is...

Which ion makes a solution basic?

How many times more acidic is a solution of pH 3 than a solution of pH 6?

Which pH value is neutral?

Which indicator would best tell apart a solution of pH 6 and one of pH 9?

Which household product should NOT be tested in a home pH inquiry?

For parents and teachers

Big idea: C3.7 asks students to describe how the pH scale classifies solutions as acidic, basic or neutral ("a solution with a pH of 1 is highly acidic; a solution with a pH of 7 is neutral"), and C2.6 asks them to plan and conduct an inquiry to classify common substances using indicators or pH strips. Grade 10 adds the ion explanation (H⁺ and OH⁻) and the ×10 meaning of each pH step, which Grade 11 chemistry builds on.

Common difficulties:

Thinking higher pH means more acidic. It is the reverse.
Thinking pH 4 is "twice as acidic" as pH 8. Each step is a factor of 10: pH 4 is 10 000 times more acidic than pH 8.
Over-reading one indicator. Colourless phenolphthalein does not prove a solution is acidic.

Safety at home: please supervise the inquiry and keep it to food and personal-care products. Do not let your teen test or handle bleach, ammonia, drain cleaner, oven cleaner or toilet cleaner, and never mix any of them. Pool or aquarium pH strips are an inexpensive, safe option; red cabbage indicator works too (you handle the knife and hot water).