74. why does ice float in water? explain using density. 75. what is the difference between a physical change…

Brief Explanations:

74. Ice floats in water because its density is less than that of water. A physical change is a change in the form or state of matter without changing its chemical composition (e.g., melting ice). A chemical change results in a new substance being formed (e.g., iron rusting).
75. The law of conservation of mass states that in a closed - system chemical reaction, the total mass of the reactants equals the total mass of the products.
76. The balanced chemical equation for $H_2 + O_2\rightarrow H_2O$ is $2H_2+O_2 = 2H_2O$.
77. An exothermic reaction releases heat to the surroundings. Example: Combustion of wood.
78. An endothermic reaction absorbs heat from the surroundings. Example: Dissolving ammonium nitrate in water.
79. A catalyst in a chemical reaction increases the rate of the reaction without being consumed in the reaction itself.
80. A transverse wave has particles vibrating perpendicular to the direction of wave propagation, while a longitudinal wave has particles vibrating parallel to the direction of wave propagation.
81. Sound travels through different materials by the vibration of particles. It travels fastest through solids, slower through liquids, and slowest through gases.
82. The frequency of a wave can be calculated using the formula $v = f\lambda$, where $v$ is the wave - speed, $f$ is the frequency, and $\lambda$ is the wavelength. Given $v = 10$ m/s and $\lambda=5$ m, then $f=\frac{v}{\lambda}=\frac{10}{5}=2$ Hz.
83. Amplitude in a wave is the maximum displacement of the particles of the medium from their equilibrium position.

Answer:

74. Ice floats because its density is less than water's. Physical change: change in form/state without new substance (e.g., melting ice). Chemical change: new substance formed (e.g., iron rusting).
75. In a closed - system chemical reaction, total mass of reactants = total mass of products.
76. $2H_2+O_2 = 2H_2O$
77. Exothermic reaction: releases heat. Example: Combustion of wood.
78. Endothermic reaction: absorbs heat. Example: Dissolving ammonium nitrate in water.
79. Catalyst increases reaction rate without being consumed.
80. Transverse wave: particles vibrate perpendicular to wave direction. Longitudinal wave: particles vibrate parallel to wave direction.
81. Sound travels through particle vibration, fastest in solids, slower in liquids, slowest in gases.
82. $f = 2$ Hz
83. Amplitude is maximum displacement of medium particles from equilibrium position.