What revolutions made the tank, machine gun, poison gas, and mortars possible?
Necessity is the mother of invention. I can come at this question from a few different angles. First, I could say the industrial revolution led to the use of interchangeable parts and mass production, making weapons in the numbers needed for these wars.
I can also argue some specific revolutions, such as the American Civil War (southerners liked to refer to it as the Second American Revolution) during which the Gatling gun became the world's first machine gun.
The tank was a revolution in tactics, slow to mature, but first developed in World War I and then perfected in the blitzkrieg style fighting of World War II. It revolutionized the way armies fought, and had fought for hundreds of years - with a mass infantry charge.
Poison gas was a revolution in warfare because it represented one aspect of the death of chivalry and the advent of total war. Its use in World War I removed any last pretense of "rules" of war.
Lastly, the invention of the mortar (much earlier) could be considered a revolution in ballistics and artillery. Small, portable, very effective and manned by a small crew, it made artillery more mobile and effective.
Advances in science and technology helped to make weapons in war more "efficient." Transpiring along the same time as industrialization, a more "productive" mindset was applied to the materials that helped to make warfighting possible. Similar to the mentality that sought to expand profit to benefit at the hands of the greater good, innovations in warfighting capacity were sought to make greater impact in terms of effectiveness. In terms of war, this meant increasing the number killed or wounded. The machine gun shot at a quicker rate, allowing greater defense and more dead. The us of poison gas was developed in laboratories and was done in the hopes of overrunning enemies with greater ease and the tank was seen as a marvel of modern industry for it was impenetrable and effective. These advances in war were brought about by the spirit of innovation and creativity that assisted most of the period of industrialization.
The simple answer to your question is trench warfare in which all of these things were employed to fight the wars. Trenches offered protection to the fighting armies from small firearms. Trench warfare by definition means both opposing armies have static lines of defense. Also, a revolution in fire power without a similar advance in mobility and communications applies here.
Some examples are the American Civil War(1861-1865), the Russo-Japanese War(1904-1905), WWI(Western Front-France and Belgium).
Poison Gas disabled and stopped enemy armies. Troops were protected from enemy small fire. Improved gas masks were made early on by urinating on a handkerchief and holding it over the nose and mouth so the urea would disable the poison.
Machine guns were revoltionary because they determined at times how an army advanced and how far. Armies basically now had a portable firearm that could be fully implemented into the battlefield. A portable cannon.
Tanks were invented and then reinvented in WWI and WWII. They basically destroyed everything. The ultimate war machine. Tanks are key in battles because strategy is used to effectively make them assist in battle. In WWI, some were being used as a form of concealment strategy because an experimental fighting machine was secretly being constructed in England. Britain is credited for the first successfu implementation of the tank in war on land.
Mortars were widely used in trenches for destruction which gave rise to the howitzer-fired a more direct arc than a mortar. Mortars were also used to hold enemy lines. They were an effective weapon of choice at close and medium range battle. Mortars complemented the tanks well especially in WWII and similar battles since then.