S2 Chemistry Quiz: pH Scale & Natural Indicators

What does the pH scale measure?

• Temperature

• How acidic or alkaline a substance is

• How concentrated a solution is

• How dangerous a chemical is

Answer explanation

The pH scale measures how acidic or alkaline a substance is, ranging from 0 (very acidic) to 14 (very alkaline), with 7 being neutral. This makes 'how acidic or alkaline a substance is' the correct choice.

What pH is neutral?

• 3

• 5

• 7

• 14

Answer explanation

A pH of 7 is considered neutral, meaning it is neither acidic nor basic. Values below 7 indicate acidity, while values above 7 indicate alkalinity. Therefore, the correct answer is 7.

Which colour shows a strong acid on universal indicator?

• Blue

• Red

• Green

• Purple

Answer explanation

Red indicates a strong acid on a universal indicator scale, while blue, green, and purple represent neutral to basic solutions. Therefore, the correct answer is red.

Which colour shows a strong alkali?

• Red

• Yellow

• Blue

• Purple

Answer explanation

Purple indicates a strong alkali in pH indicators. In the pH scale, purple represents a high pH level, typically above 11, which is characteristic of strong alkalis.

Substances with pH 0–6 are called:

• Alkalis

• Metals

• Acids

• Indicators

Answer explanation

Substances with pH 0–6 are classified as acids. This range indicates increasing acidity, with lower pH values representing stronger acids. Therefore, the correct answer is acids.

True/False: A substance with pH 14 is a strong alkali.

• True

• False

Answer explanation

A substance with a pH of 14 indicates a very high concentration of hydroxide ions, characteristic of strong alkalis. Therefore, the statement is true.

True/False: A substance with pH 7 is neutral.

• True

• False

Answer explanation

A substance with a pH of 7 is considered neutral, meaning it is neither acidic nor basic. Therefore, the statement is True.

Which of the following is an acid?

• Soap

• Toothpaste

• Vinegar

• Baking powder

Answer explanation

Vinegar is an acid, specifically acetic acid, which gives it its sour taste. In contrast, soap and toothpaste are basic, while baking powder is a neutralizing agent, not an acid.

Which of the following is an alkali?

• Lemon juice

• Cola

• Vinegar

• Soap

Answer explanation

Soap is an alkali because it has a high pH level, making it basic. In contrast, lemon juice, cola, and vinegar are acidic, with lower pH values.

Universal indicator turns green when a substance is:

• Acidic

• Neutral

• Alkaline

• Very strong acid

Answer explanation

Universal indicator turns green at a pH of around 7, which indicates a neutral solution. Acidic solutions turn red, while alkaline solutions turn blue. Therefore, the correct answer is neutral.