Acids, Bases and Salts: 10th Science

Question: What is meant by p and pH?

Answer: p in pH stands for ‘potenz’ in Germany and H is for hydrogen.

Question: What are antacids? Explain their role in providing relief from stomach ache.

Answer:

1. Antacids are mild base.
2. Antacids neutralise the effect of extra acid produced in the stomach during indigestion and thus provide relief.

Question: How does the change occur in values of milk when it changes to curd?

Answer: When milk changes to curd, it becomes acidic due to the formation of lactic acid. Thus, its pH value decreases and becomes less than 6.

Question: Define alkalies and give an example.

Answer: The bases soluble in water are called alkalies. e.g., NaOH, KOH.

Question: Mention the range of pH value for a identification of a base.

Answer: pH value for identification of a base is 7-14.

Question: How chloride of lime differs chemically from calcium chloride?

Answer: Chloride of lime is CaOCl₂ while calcium chloride is CaCl₂.

Question: What is meant by “water of crystallization” in a substance? Explain with an example.

Answer: Water of crystallization the fixed number of water molecules present in one formula unit of a salt.
Example: Hydrated copper sulphate (CuSO₄.5H₂O). Here, 5H₂O signify five molecules of water of crystallization.

Question: Write the chemical name and chemical formula of washing soda.

Answer: Chemical name: Sodium carbonate
Chemical formula: Na₂CO₃.10H₂O

Question: What effect does an increase in concentration of H⁺(aq) in a solution have on the pH of solution?

Answer: The pH of a solution decrease with the increase in the concentration of H⁺(aq) in a solution. More are the H⁺(aq) in a solution, more is the acidity, and less is the pH value of that solution.

Question: Name the gas evolved when dilute HCl reacts with sodium hydrogen carbonate. How is it recognized?

Answer: The gas evolved is CO₂. When this gas is passed through lime water, it turns it milky.

Question: What are olfactory indicators?

Answer: The substances whose odour changes in acidic or basic media are called olfactory indicators.

Question: Why does 1 MHCl solution have a high concentration of H⁺ ions than 1 M CH₃COOH solution?

Answer: HCl solution is a strong acid and, therefore, ionises completely as compared to CH₃COOH solution which is a weak acid. Due to complete ionisation, it contains high concentration of H⁺ ions. ₂

Question: Which one of the following has a higher concentration of  H⁺ ions? 1 M HCl or 1 M CH₃COOH.

Answer: 1 M HCl has higher concentration of H⁺ ions. Being strong acid, it ionises completely to furnish H⁺ ions.

Question: How will you test for the gas which is liberated when hydrochloric acid reacts with an active metal?

Answer: When burning candle is brought near to the jar filled with hydrogen gas, it burns explosively with a pop sound.

Question: In which form H⁺(hydrogen) ions exist in water?

Answer: Since hydrogen ions cannot exist alone, therefore, they exist after combining with water molecules. Thus, hydrogen ions must be shown as H⁺ (aq) or hydronium ion (H₃O⁺).

Question: How is the pH of a solution of an acid influence when it is diluted?

Answer: On dilution, the concentration of hydrogen ions per unit volume decreases and hence pH of the solution increases. Since pH is the negative logarithmic value of the concentration of hydrogen ions.

Question: At what pH rain water is said to be acidic?

Answer: When pH of rain water is less than 5.6, then it is called acid rain.

Question: Which gas is evolved when dilute hydrochloric acid reacts with zinc? Write the molecular formula of this gas.

Answer: Hydrogen gas. Molecular formula is H.

Question: Dry HCl gas does not change the colour of dry litmus. Give reason.

Answer: H⁺ ion from HCl cannot ionise to give H⁺ ions in absence of water. Acidic property like change in colour of litmus depends on production of H⁺ ion, hence there is no colour change.

Question: Why is HCl a stronger acid than acetic acid?

Answer: On dissociation, it yields larger [H⁺] Hydrogen ions for the same concentration as compared to acetic acid.

Question: Tooth enamel is the hardest substance in our body. Name the compound of which it is made-up of. At what pH of the mouth it gets corroded? State the role of bacteria present in the mouth. Suggests a method to present tooth decay.

Answer: Calcium phosphate; below 5.5; bacteria produce acids by degradation of sugar and food particles remaining in the mouth after eating; using tooth pastes which are generally basic.