Ch.9.2 Flashcards
Why do we have indicators?
This is because many acids and alkalis used in the laboratory are colourless, corrosive liquids. Hence, we cannot distinguish them based on their appearances, or by tasting or touching. Therefore, we can use indicators to show the presence of acids and alkalis.
Where do many acid-alkali indicators come from?
They are extracted from plants in Nature.
Give an example of a natural indicator and explain why it can be used as one.
Red cabbage extract. It can be used as a natural indicator because red cabbage contains pigments called anthocyanins, which change colour in the presence of an acid or alkali.
Name 5 examples of plants that contain the same pigment as red cabbage extracts.
Red rose flower, blueberry, strawberry, red radish and butterfly pea flowers.
Can all plant extracts be used as an indicator for distinguishing acids and alkalis? Why?
No, because not all plants contain pigments which change colour in acids and alkalis.
Name one plant which contains pigments that are not those found in red cabbages,, but also change colour in the presence of an acid or an alkali. Name the pigment.
Beetroot,, betalains.
Apart from natural indicators,, name one commonly used alternative that can be used as an acid-alkali indicator.
Litmus.
What is litmus composed of?
It is a mixture of pigments extracted from lichen.
Why can litmus distinguish between acidic and alkaline solutions?
This is because it shows different colours in acidic and alkaline conditions.
Litmus is available in two forms and two colours::
forms: : __________________ and __________________
colours: : __________________ and __________________
solution,, paper; red,, blue
How is litmus paper prepared?
They are prepared by soaking strips of filter paper in litmus solution and then drying them before use.
__________________/ __________________ litmus is used for testing __________________/ __________________ solutions.
blue + acidic
red + alkaline
Acidic solutions turn _____________ litmus _____________.
Alkaline solutions do the opposite.
blue,, red.
How do we know if a solutions is neutral?
If the solution doesn’t react to either litmuses, it is neutral.
What is the major disadvantage of using natural indicators and/ or litmus?
They can only help distinguish whether a substance is acidic or alkaline. They cannot show how acidic or alkaline a substance is.
Name the indicator that we should use when we want to know how acidic or alkaline a substance is.
A universal indicator.
What is a universal indicator composed of?
It is a mixture of several indicators.
How does a universal indicator work?
It shows a range of different colours depending on the degrees of acidity or alkalinity of the substance mixed with it.
Universal indicator is available in _________________ and ________(a)________ forms. The (a) form of universal indicator is called __________________________.
solution, paper; pH paper.
There is a colour chart on the _______________________________________ (for liquid form) or the _______________________________________ (for other form). Each colour on the chart _______________________________________.
bottle of universal indicator solution, box of pH paper, corresponds to a pH value
The pH value is expressed as _______________________________________. This range of pH values is known as the _________________________.
a number from 0 to 14, pH scale.
On the range of pH values:
—an acidic substance has a pH value ____________________ :: the ____________________ the pH value,, the more acidic the substance.
—an alkaline substance has a pH value ____________________ :: the ____________________ the pH value,, the more alkaline the substance.
—neutral substances have a pH value of ____________________.
less than 7,, lower,, greater than 7,, higher,, exactly 7.
pH values of common substances:: 0 — 1 — 2 — 3 — 4 — 5 — 6 — 7 — 8 — 9 — 10 — 11 — 12 — 14 —
acid in car battery sulphuric acid cola orange juice / vinegar tomato juice black coffee tap water / tea distilled water toothpaste detergent baking soda glass cleaner limewater drain cleaner
What is the full name of pH?
power of Hydrogen.
Is pH value of less than 0 or greater than 14 possible? Do pH values have to be whole numbers?
Yes. No.
Universal indicator can give only a ________________________________ of the pH value of a substance. What can we do to measure pH values more accurately?
rough measurement; we can use an electronic instrument — pH meter.
What does a pH meter consist of?
It consists of a probe connected to a digital display.
What is the accuracy of a pH meter?
It is accurate to two decimal places.
What are the steps to using a pH meter?
1.
2.
3.
- calibrate the pH meter
- put its probe into a solution
- the pH value of the solution will be displayed on its screen
Besides the pH meter, we can use a _______________________________________________ to measure the pH value of a solution.
pH sensor connected with a data-logger.
What is the benefit of using a data-logger?
The readings from the data-logger can be recorded and analysed in a computer. This way, the data-logger system can help monitor the change in pH value of a solution during an experiment.
Name three advantages of a pH meter (compared to universal indicator solution and/ or pH paper).
—a pH meter can measure the pH value of a substance more accurately than universal indicator solution and pH paper
—a pH meter can be reused while pH paper cannot
—a pH meter can be used to measure the pH value of a liquid with a deep colour
Name four disadvantages of a pH meter (compared to universal indicator solution and/ or pH paper).
—a pH meter is more expensive than universal indicator solution and pH paper
—the size of a pH meter is larger than that of pH paper. It is less convenient to carry around
—calibration is needed, making it less user friendly and more difficult to operate.
—we need to handle pH meters with care, its probe is made of glass.
