Exploring Substances: Acidic, Basic, and Neutral – Case-based Questions with Answers

Ans. (a) Lemon will be strongly acidic, tomato will be mildly acidic, and cucumber will be almost neutral because it does not taste sour.

Ans. (b) Many laboratory acids are strong and corrosive. Tasting them can burn the tongue, damage internal organs or even be poisonous. Therefore, unknown chemicals must never be tasted; instead, we use indicators.

Ans. (c) Lemon contains the natural acid called citric acid.

Ans. (a) Substances that taste bitter and feel soapy are usually bases.

Ans. (b) Both soap solution and baking soda solution are basic in nature.

Ans. (c) Many bases, including concentrated soap and detergents, can irritate or damage the eyes because they may be mildly corrosive. That is why we should protect our eyes and wash them with clean water if soap enters.

Ans. (a) Sugar solution and common salt solution are neutral because they do not change the colour of either red or blue litmus paper.

Ans. (b) Lemon juice is acidic (turns blue litmus red), and soap solution is basic (turns red litmus blue).

Ans. (c) Many body fluids like blood are nearly neutral. If they become too acidic or too basic, organs cannot function properly. Neutral substances help maintain this balance and keep us healthy.

Ans. (a) P is acidic (blue → red), Q is basic (red → blue), R is neutral (no change), and S shows both changes, suggesting a mixture or strong solutions tested separately.

Ans. (b) S shows both types of change in the chart, which may indicate that it is not a single solution, or that the observation is a combined record of a strong acid and a strong base. In real lab work, one solution cannot be acidic and basic at the same time.

Ans. (c) Litmus is obtained from lichens, which are natural organisms. Since it is derived from natural sources and shows different colours in acids and bases, it is called a natural indicator.

Ans. (a) Turmeric turns reddish-brown in the presence of a base. Since soap water caused this change, soap is basic in nature.

Ans. (b) Lemon juice is acidic. Turmeric remains yellow in acidic or neutral solutions, so no major colour change is seen. Turmeric mainly shows colour change with bases.

Ans. (c) Turmeric is easily available in the kitchen, safe to use and gives a clear colour change with bases. Therefore, it is a convenient natural indicator for home experiments.

Ans. (a) Vinegar, which turns the indicator red, is acidic. Soap solution, which turns it greenish, is basic. Sugar solution, causing little change, is neutral.

Ans. (b) Sugar solution is neutral, so it does not change the colour of china rose indicator significantly. Therefore, the indicator does not show it as acidic or basic.

Ans. (c) This activity shows that plant-based indicators like china rose are simple to prepare, safe to use and can clearly distinguish between acids and bases through colour changes.

Ans. (a) The extra substance is acidic, because excess hydrochloric acid has been produced in the stomach.

Ans. (b) Milk of magnesia is a mild base (it contains magnesium hydroxide). It is given to neutralise the extra acid and relieve the burning sensation.

Ans. (c) The process is called neutralisation – a reaction between an acid and a base to form salt and water, reducing the effect of both.

Ans. (a) Many plants cannot grow well in highly acidic soil. Essential nutrients may dissolve and wash away or become unavailable to plants, reducing crop growth.

Ans. (b) Quicklime and slaked lime are basic substances.

Ans. (c) When added to acidic soil, these bases neutralise the extra acid. This increases the soil pH towards neutral, improving soil quality and helping plants grow better.

Ans. (a) The substance injected by a bee sting is acidic.

Ans. (b) Baking soda is a mild base. It neutralises the acid injected by the bee, reducing pain and irritation.

Ans. (c) The reaction is of neutralisation type: Acid + Base → Salt + Water.

Ans. (a) The chemical injected by a wasp sting is usually basic.

Ans. (b) Vinegar contains acetic acid, a weak acid. It is used to neutralise the basic venom from the wasp sting, reducing pain.

Ans. (c) This example shows that we must know whether the sting is due to an acid or a base. Only then can we choose the correct opposite type (acid or base) to neutralise it. Wrong choice may worsen the problem.

Ans. (a) The gases form acids with rainwater, producing acid rain.

Ans. (b) Acid rain makes the lake water more acidic and lowers its pH below normal.

Ans. (c) Many aquatic organisms, including fish, can survive only in water with a limited pH range. Excess acidity harms their body functions and may kill them, causing the observed deaths.

Ans. (a) The main gas pollutants are sulphur dioxide (SO₂) and nitrogen oxides (NOₓ).

Ans. (b) Acids in acid rain react with calcium carbonate present in marble and dissolve it slowly. This causes the surface to lose its shine and fine carvings to get damaged.

Ans. (c) (i) Reduce air pollution by using cleaner fuels and controlling factory emissions.
(ii) Encourage public transport and tree plantation to reduce pollutants that cause acid rain.

Ans. (a) Acid rain has made the soil more acidic, which is harmful for plants.

Ans. (b) Limestone (calcium carbonate) is basic in nature.

Ans. (c) Limestone neutralises the extra acid in the soil. This brings the soil pH closer to neutral, making conditions better for plant growth and improving crop yield.

Ans. (a) Lemon juice (pH 2) – acidic; Distilled water (pH 7) – neutral; Soap solution (pH 10) – basic; Milk (pH 6.5) – slightly acidic.

Ans. (b) Lemon juice is the strongest acid among them because it has the lowest pH value (2), which means high acidity.

Ans. (c) Distilled water has pH 7, the middle of the pH scale. It is neither acidic nor basic, so it is considered neutral.

Ans. (a) Many chemical reactions in our body work properly only in a narrow pH range. A large change in blood pH can disturb the working of organs like heart and brain, so it must remain almost constant.

Ans. (b) If stomach pH becomes too low, the stomach becomes too acidic. This causes indigestion, pain and burning sensation (acidity).

Ans. (c) Antacids are basic substances. They neutralise the extra acid in the stomach, raising the pH closer to normal and giving relief from acidity.

Ans. (a) Acids produced by bacteria dissolve the hard, outer enamel of the teeth. Over time this leads to tooth decay and cavities.

Ans. (b) A basic toothpaste neutralises the acids in the mouth, preventing further damage to the enamel and protecting the teeth.

Ans. (c) Brushing teeth twice a day and rinsing the mouth after eating, especially after sweets, reduces the time acids remain in the mouth and limits tooth decay.

Ans. (a) The basic substance is quicklime (calcium oxide), which is converted to slaked lime (calcium hydroxide) with water.

Ans. (b) It reacts with carbon dioxide present in the air.

Ans. (c) The final compound formed is calcium carbonate, which sticks to the walls and gives a smooth white appearance.

Ans. (a) Concentrated acids give off strong fumes that can irritate or damage the nose and lungs. Direct smelling may cause serious injury.

Ans. (b) Acid and base bottles should be carried carefully with both hands, keeping them close to the body and the cap tightly closed.

Ans. (c) The affected area should be washed immediately with plenty of clean water, and the teacher should be informed at once.

Ans. (a) When acid is mixed with water, a chemical reaction takes place which releases heat. This is an exothermic process.

Ans. (b) Pouring water into concentrated acid can cause sudden heating and boiling. The acid may splash out of the container and cause burns.

Ans. (c) The correct rule is: “Always add acid to water, never water to acid.”

Ans. (a) Science explains that stomach pain can be caused by excess acid (acidity) in the stomach, which irritates its inner wall.

Ans. (b) The teacher can mix a few drops of dilute acid with an antacid solution in a test tube using an indicator. The change in colour and reduction in acidity show neutralisation, similar to what happens in the stomach.

Ans. (c) This example encourages people to look for logical, evidence-based explanations instead of superstitions. It shows that understanding science can help us take correct decisions about health and treatment, which is a sign of scientific attitude.