Grade 7Integrated Science

Acids, Bases and Indicators

Properties of acids and bases; pH scale; natural and synthetic indicators.

📖 4 min read · 3 worked examples · 6 practice questions

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The lesson

By the end of today, you'll be able to describe their properties, explain the pH scale, and identify both natural and synthetic indicators. First, notice the topic line: Acids, Bases & Indicators. We'll connect these ideas to everyday Kenyan examples—like the sour taste of lemon juice, the tang of fermented milk, and the vibrant colors from local plant extracts such as beetroot. Keep these goals in mind as we move forward: describe properties, explain pH, and identify indicators. If anything feels unclear, just raise your hand and we'll pause to check understanding.

Class, let's dive into today's topic: What Are Acids? First, acids are substances that release hydrogen ions, H⁺, when they dissolve in water. This release is what gives them their characteristic behavior. You'll notice on the slide that acids taste sour, turn blue litmus paper red, and they react with metals, often producing bubbles of hydrogen gas. Here are some Kenyan examples: the juice from a lemon and tamarind concentrate are both acidic, which is why they taste tangy and can be used to preserve foods. Any questions so far? Feel free to raise your hand or share an example of something acidic you've encountered at home.

Everyone, let's explore what bases are. A base is a substance that can release hydroxide ions when it dissolves in water. First, notice that bases release hydroxide ions (OH⁻) in water. This is the chemical reason they behave the way they do. Second, think about their properties: they taste bitter, feel slippery, and turn red litmus paper blue. Those are clues you can use in the lab. Finally, here are two Kenyan examples: a soap solution and a baking‑soda solution. Both are basic because they contain substances that release hydroxide ions. To recap, bases give off OH⁻ ions, have a bitter‑slippery feel, change litmus blue, and you can find them in everyday items like soap and baking soda.

Everyone, let's bring everything together with a quick recap of what we've learned about acids, bases and indicators. First, remember that acids release hydrogen ions (H⁺) and bases release hydroxide ions (OH⁻). Those ions give each substance its characteristic properties – acids taste sour and can corrode metal, while bases feel slippery and feel bitter. Next, the pH scale runs from 0 to 14. A pH of 0 means a very strong acid, 7 is neutral – like pure water – and 14 signals a very strong base. Think of it as a ruler that measures how acidic or basic something is. We also saw natural indicators such as red cabbage or beetroot, which change colour depending on the pH, and synthetic ones like phenolphthalein that turn pink in basic solutions. Finally, you can apply this knowledge at home or in the field – testing soil acidity for farming, checking the freshness of food, or even making safe cleaning solutions with common kitchen items. That's the big picture: acids give H⁺, bases give OH⁻, the pH scale tells us how strong they are, and indicators help us see those differences in everyday life. Great job, everyone!

Worked examples

Lemon Juice

Let's dive into our worked example titled Worked Example 1: Lemon Juice. We'll see how to calculate the acidity of a familiar Kenyan drink. First, the problem gives us a 0.01 M solution of citric acid in lemon juice. Our task is to find the hydrogen‑ion concentration, [H⁺], and then the pH. We use the formula [H⁺] = √(Kₐ × C). For citric acid, Kₐ is about 7.4 × 10⁻⁴. Plugging in the concentration (C = 0.01 M) gives [H⁺] ≈ √(7.4 × 10⁻⁶) ≈ 2.7 × 10⁻³ M. PH = –log[H⁺]. Taking the negative logarithm of 2.7 × 10⁻³ yields a pH of roughly 2.6, which tells us lemon juice is quite acidic—well within the acidic region of the pH scale. To recap, we started with a 0.01 M citric acid solution, applied the √(Kₐ C) relationship, found [H⁺] ≈ 2.7 × 10⁻³ M, and calculated a pH of about 2.6. Any questions before we move on?

Soap Solution

Class, let's work through Example 2, which looks at a typical soap solution you might find in a Kenyan home. First, we're told the soap contains 0.02 M sodium hydroxide. That's the concentration of the base dissolved in the water. To find the hydroxide ion concentration, [OH⁻], we simply note that NaOH dissociates completely, so [OH⁻] equals the original 0.02 M. Next we calculate pOH using the formula pOH = –log[OH⁻]. Plugging in 0.02 gives pOH ≈ 1.70. Since pH + pOH = 14, the pH of the soap solution is 14 – 1.70 ≈ 12.3, which is quite basic. A basic solution feels slippery because the OH⁻ ions reduce friction on our skin – that's why soap helps us clean. To recap: we started with 0.02 M NaOH, found [OH⁻] = 0.02 M, calculated pOH ≈ 1.70, and determined a pH of about 12.3, explaining the slippery feel of soap.

Red Cabbage Indicator

Class, let's dive into Worked Example 3 where we use red cabbage as a natural pH indicator to identify an unknown solution. First, we extract the pigment from the cabbage and observe the colour change when it reacts with the unknown liquid. Notice how the colour shifts—this tells us about the acidity or basicity of the solution. Next, we match the observed colour to the pH colour chart on the slide to determine the pH range. Finally, based on that range we can conclude whether the unknown is acidic, neutral, or basic. Any questions before we try it out?

Practice questions

First, think about what makes a substance a base. Recall that bases usually produce hydroxide ions (OH‑) when they dissolve in water, and they often feel slippery or taste bitter.
Remember what a neutral solution looks like. A neutral solution has a pH of 7, and it contains equal amounts of hydrogen ions (H⁺) and hydroxide ions (OH⁻).
Finally, visualize the universal indicator color chart. At pH 3 the indicator turns a bright red‑orange, leaning toward red.
Remember the cabbage leaf test we saw earlier: when the leaf turns pink, it means the solution is acidic, so the soil pH is less than 7. That's why option A is the correct answer for the first question.
For the second question, think about everyday items that change colour in acidic or basic solutions. Red cabbage leaves are a classic natural indicator, and lemon juice is acidic, so both of those will show a colour change.
Finally, blue litmus paper stays blue in basic solutions and turns red in acidic ones. Since a pH of 9 is basic, the paper will stay blue, making option B the correct answer.

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Acids, Bases and Indicators

📖 The lesson

✏️ Worked examples