SNC2D Acids, Bases, pH and Indicators: Notes and Practice

Lemon juice, vinegar and cola are acidic. Baking soda, soap and window cleaner are basic. Your stomach makes hydrochloric acid strong enough to react with some metals, while your blood is held at a slightly basic pH of about 7.4, and a drift of even a few tenths of a unit is a medical emergency. Gardeners test soil pH, pool owners test water pH every week, and water treatment plants adjust it around the clock. This lesson explains what acids and bases are, how the pH scale measures them, and how to test a substance safely.

Properties of acids and bases

AAcids

pH less than 7
Turn blue litmus paper red
React with active metals such as magnesium and zinc to release hydrogen gas
React with carbonates (like limestone or baking soda) to release carbon dioxide
Conduct electricity when dissolved in water
Examples: hydrochloric acid, vinegar, citrus juice

BBases

pH greater than 7
Turn red litmus paper blue and phenolphthalein pink
Usually do not react with metals or carbonates
Historically described as slippery, because strong bases react with the oils in skin (never touch them to test)
Conduct electricity when dissolved in water
Examples: sodium hydroxide, ammonia, baking soda solution

Fig 1A side-by-side comparison of acids (pH below 7) and bases (pH above 7): their effect on litmus and phenolphthalein, their reactions with metals and carbonates, their electrical conductivity and the ions they release in water. The traditional taste and feel are listed too, but a footnote warns never to taste or touch lab chemicals: identify acids and bases with indicators and pH.

Never taste, touch or sniff to identify

Acids were once identified by their sour taste and bases by their bitter taste and slippery feel, but tasting or touching an unknown chemical can cause serious burns. Strong acids and bases are corrosive and carry the WHMIS corrosion pictogram. Use indicators or a pH meter, wear safety goggles, and if a chemical touches your skin or eyes, rinse with plenty of running water.

What makes something an acid or a base?

The Swedish chemist Svante Arrhenius explained these properties in the 1880s. An acid is a substance that releases hydrogen ions, $H^{+}$ , when it dissolves in water. A base is a substance that releases hydroxide ions, $OH^{-}$ , in water. The more of these ions in a given volume of solution, the more strongly acidic or basic it is. Pure water contains tiny, equal amounts of both, so it is neutral. A few bases, like ammonia ( $NH_{3}$ ), do not contain OH in their formula; they make hydroxide ions by reacting with water.

HCl(aq) \to H^{+} (aq) + Cl^{-} (aq) NaOH(aq) \to Na^{+} (aq) + OH^{-} (aq)

Hydrochloric acid releases hydrogen ions; sodium hydroxide releases hydroxide ions.

The pH scale

The pH scale runs from about 0 to 14. A pH below 7 is acidic, exactly 7 is neutral, and above 7 is basic. The lower the pH, the more acidic; the higher the pH, the more basic. The key idea is that the scale is logarithmic: each step of 1 pH unit means a factor of 10 in the concentration of hydrogen ions. Lemon juice at about pH 2 is 10 times more acidic than vinegar at about pH 3, and 100 times more acidic than tomato juice at about pH 4. A change of a few pH units is therefore a very big change.

how many times more acidic = 1 0^{(higher pH - lower pH)}

Example: pH 3 compared with pH 6 gives 10³ = 1000 times more acidic.

Fig 2The pH scale from 0 to 14 with household substances placed approximately: battery acid, stomach acid, lemon juice, vinegar and cola on the acidic side; coffee and milk slightly acidic; pure water at 7; blood, seawater, baking soda, ammonia cleaner, bleach and oven cleaner on the basic side. Each step of 1 is a 10× change. Litmus is red in acid and blue in base; phenolphthalein is colourless in acid and pink above about pH 8.2.

Indicator	Colour in acid	Colour in base	Changes near
Litmus paper	red	blue	a broad range centred near pH 7 (red litmus tests for bases, blue litmus for acids)
Phenolphthalein	colourless	pink	pH 8.2 (stays colourless in neutral solutions)
Bromothymol blue	yellow	blue	green between about pH 6 and 7.6
Universal indicator or pH paper	red, orange, yellow	blue, purple	a full colour range matched to a chart; green near 7
Red cabbage juice	red to pink	blue-green to yellow	purple near neutral; a natural indicator you can make at home

Common acid–base indicators. Litmus only tells you acid or base; pH paper and pH meters give an approximate number.

Put on safety goggles. Dissolve solids such as baking soda in a little distilled water first.
Dip a clean glass rod in the substance and touch it to a strip of red litmus and a strip of blue litmus.
If blue litmus turns red, the substance is acidic; if red turns blue, it is basic; if neither changes, it is close to neutral.
Touch the rod to a strip of pH paper and compare the colour with the chart to estimate the pH.
Rinse the rod with distilled water between samples so one substance does not contaminate the next.

pH on the job

Water treatment operators check the pH of drinking water constantly and adjust it. Health Canada's guidance is to keep treated drinking water between about pH 7.0 and 10.5, mainly to protect pipes. Water that is too acidic corrodes pipes and can pick up metals such as lead and copper; water that is too basic tastes bitter and leaves scale. Pool technicians, farmers, brewers and lab technicians also rely on pH measurements every day.

Acids, Bases, pH and Indicators

Key things to remember

Properties of acids and bases

What makes something an acid or a base?

The pH scale

Worked examples

Where marks get lost

Flashcards 8

Practice