10 - Science - 11. Chemistry – Chemical Reactions

Explanation
The given reaction involves the displacement of hydrogen from hydrochloric acid by zinc. This is evident from the formation of zinc chloride and the evolution of hydrogen gas. Therefore, the reaction is an example of a displacement reaction.

Explanation
When the reddish brown element X is heated in air, it forms a black compound Y. The correct answer is Cu, CuO because copper (Cu) is a reddish brown element and when it is heated in air, it reacts with oxygen to form copper oxide (CuO), which is a black compound.

Explanation
When a pH paper shows a green color, it indicates that the solution is neutral, which is the case with pure water. Lemon juice, on the other hand, is acidic in nature. Therefore, when the student adds lemon juice to the water and tests it with the pH paper, he will observe a red color. This is because the pH paper will indicate that the solution is acidic, and red is the color associated with acidic solutions on pH papers.

Explanation
A chemical volcano is an example of a decomposition reaction. In a decomposition reaction, a single compound breaks down into two or more simpler substances. In the case of a chemical volcano, a mixture of chemicals react to produce a gas, which causes the eruption and the release of heat and smoke. This reaction involves the decomposition of the chemicals present in the volcano mixture, resulting in the formation of new substances and the release of energy.

Explanation
When crystals of lead nitrate are heated strongly, they undergo decomposition and produce NO2 gas. NO2 gas is reddish brown in color, hence the color of the gas is reddish brown.

Explanation
When aqueous solutions of silver nitrate and sodium chloride are mixed, a white precipitate is immediately formed. This is because silver nitrate and sodium chloride react to form silver chloride, which is insoluble in water and therefore precipitates out. The white color of the precipitate is due to the presence of silver chloride.

Explanation
Zinc is more reactive than aluminium. This means that zinc has a higher tendency to undergo a chemical reaction compared to aluminium. In this case, zinc can displace aluminium metal from an aqueous solution of aluminium sulphate. This displacement reaction occurs because zinc can easily react with the sulphate ions in the solution, causing the aluminium metal to be displaced and forming zinc sulphate.

Explanation
To protect tooth decay, we are advised to brush our teeth regularly. The toothpaste commonly used is basic in nature. This is because basic toothpaste helps to neutralize the acidic environment in the mouth, which is caused by the bacteria that feed on sugars and produce acids. By using a basic toothpaste, it helps to balance the pH level in the mouth and prevent tooth decay.

Explanation
Curd contains lactic acid. This is because lactic acid is a byproduct of the fermentation process that occurs when bacteria break down lactose, the sugar found in milk. As curd is made from milk, it naturally contains lactic acid.

Explanation
The pH of a solution is determined by the concentration of hydrogen ions (H+). The pH scale is logarithmic, meaning that each unit represents a tenfold difference in acidity or alkalinity. In this case, the given hydrogen ion concentration is 0.001M, which is a relatively low concentration. Taking the negative logarithm of 0.001 gives a pH of 3, indicating that the solution is acidic.